Patent Description:
A wind turbine converts kinetic energy of the wind into electrical energy. A generator converts the wind energy captured by a rotor having one or more rotor blades into electrical energy that is usually supplied to a utility grid. The generator is housed in a nacelle together with the various components required to operate and optimize the performance of the wind turbine. A tower supports the load presented by the nacelle and the rotor. In a horizontal axis wind turbine (HAWT) the rotor blades extend radially outwardly from a central hub which rotates about a longitudinal axis aligned generally horizontally. In operation, the blades are configured to interact with the passing air flow to produce lift that causes the rotor to rotate within a plane substantially perpendicular to the direction of the wind. A conventional rotor blade is made from an outer shell and one or more inner spars in a hollow space bounded by the outer shell. The spar serves to transfer loads from the rotating blade to the hub of the wind turbine. Such loads include tensile and compressive loads directed along the length of the blade arising from the circular motion of the blade and loads arising from the wind which are directed along the thickness of the blade, i.e. from the windward side of the blade to the leeward side. The spar may typically have a hollow tubular section, e.g. generally rectangular hollow tubular section, or a beam section, e.g. I-beam, C-beam, H-beam, Y-beam, X-beam, etc., with one or more shear webs extending between spar caps. The spar caps may be incorporated into the outer shell or may be attached to the outer shell.

The spar cap may include pultruded fibrous strips of material. Pultrusion is a continuous process similar to extrusion, wherein fibres are pulled through a supply of liquid resin and then heated in an open chamber where the resin is cured. The resulting cured fibrous material is of constant cross section but, since the process is continuous, the material once formed may be cut to any arbitrary length. <CIT> describes a wind turbine blade with an elongate reinforcing structure comprising a stack of pultruded fibrous composite strips. The pultruded fibres are carbon fibres and extend almost the full length of the blade from root to tip.

<CIT> discloses a wind turbine rotor blade with an equipotential bonding, wherein the blade comprising spar caps including a plurality of stack layers of a first conductive material and at least one intermediate layer, wherein the layers of the first conductive material include a first portion of a second conductive material and a second portion of glass fibre.

The general trend in the wind turbine industry over the past several years has been for wind turbine blades to be made longer. The longer the blades, the larger the rotor and the more wind energy can be captured, improving the efficiency of an individual wind turbine.

Wind turbines are susceptible to lightning strikes. It is common for a wind turbine to include a lighting protection system, which electrically couples the wind turbine components to the ground. The blades, and in particular the blade tips, are particularly susceptible to lighting strikes. The blades therefore typically include a metallic foil, or surface protection layer (SPL), incorporated into the outer shell near the outer surface of the blade. The metallic foil may cover only a portion or substantially the entire blade outer surface. Additionally, or alternatively, the lightning protection system may include one or more discrete lightning receptors. The metallic foil and/or the lightning receptors on the blade are all electrically connected through the tower to ground.

Although lightning strikes have a natural tendency, due to their high frequency, to follow along the outside surface of a structure, such as the rotor blade, the presence of conductive fibres in the blade shell may cause unwanted discharges when a lightning strike occurs that could cause damage to the blade shell. This problem may be exacerbated where the conductive fibres extend along a substantial length of the rotor blade, especially for a long blade.

In order to reduce the prospect of high electrical potentials developing between adjacent strips of pultruded material, a conductive interleave may be arranged between the strips of pultruded material in order to equipotentially bond the strips. However, where the conductive interleave is a fabric material, conductive fibres may separate from the bulk of the fabric material during manufacture and may thereby create undesirable electrical coupling to nearby parts.

A first aspect of the invention provides a wind turbine rotor blade spar cap, the spar cap having a length and comprising: a stack comprising a plurality of layers of conductive material and at least one intermediate layer, wherein the layers of conductive material each have a length along the length of the spar cap in a first direction, wherein the intermediate layer is arranged between adjacent layers of the conductive material, wherein the intermediate layer includes a fibre fabric material having: a first edge extending in the first direction, a conductive portion having conductive fibres oriented in the first direction, a first border portion between the first edge and the conductive portion, the first border portion having a plurality of non-conductive fibres oriented in the first direction and no conductive fibres oriented in the first direction, and cross fibres oriented to cross the conductive fibres and the non-conductive fibres, and wherein the intermediate layer is bonded with the adjacent layers of the conductive material and is electrically coupled to the adjacent layers of conductive material so as to equipotentially bond the adjacent layers of the conductive material via the conductive portion of the intermediate layer.

Overall, the fibre fabric material may act to promote the infusion of resin matrix material between the strips of conductive material in order to bond the strips of conductive material together. By providing a conductive portion of the fibre fabric material, the adjacent layers of conductive material may be equipotentially bonded such that undesirable discharges or arcs within the spar cap may be avoided. The border portion may provide a buffer so that, in the case that fibres separate from the bulk material during resin infusion of the blade material, or during layup of the blade material, as loose fibres, the loose fibres are likely to be non-conductive and so are unlikely to create undesirable electrical connections.

The first border portion may have a width perpendicular to the first direction, the width of the first border portion being at least <NUM> millimetres.

The conductive fibres may be carbon fibres.

All conductive fibres of the fibre fabric material may be oriented in the first direction.

The non-conductive fibres may be glass fibres. In other examples they may be natural fibres.

The cross fibres may be non-conductive cross fibres, optionally the non-conductive cross fibres may be glass fibres or non-conductive natural fibres.

The cross fibres may be oriented perpendicular to the first direction. In another example, the cross fibres may be oriented at an angle to the first direction, such as at plus or minus <NUM> degrees.

The conductive material may comprise pultruded fibrous composite material, preferably carbon fibre reinforced plastic.

The wind turbine rotor blade spar cap may further comprise alternating layers of the conductive material and the intermediate layer.

The fibre fabric material may further comprise a second edge oriented in the first direction, the second edge being opposite the first edge, and a second border portion between the second edge and the conductive portion, the second border portion having a plurality of non-conductive fibres oriented in the first direction and no conductive fibres oriented in the first direction.

The second border portion may have a width perpendicular to the first direction, the width of the second border portion may be at least <NUM> millimetres.

The fibre fabric material may be woven or stitched.

According to a second aspect of the invention, there is provided a wind turbine rotor blade including at least one wind turbine rotor blade spar cap according to the first aspect.

A third aspect of the invention provides a method of manufacturing a wind turbine rotor blade spar cap, comprising: providing a plurality of layers of conductive material, each layer having a length along a length of the spar cap in a first direction; placing an intermediate layer between adjacent layers of the conductive material so as to form a stack, the intermediate layer including a fibre fabric material having: a first edge extending in the first direction; a conductive portion having conductive fibres oriented in the first direction, a first border portion between the first edge and the conductive portion, the first border portion having a plurality of non-conductive fibres oriented in the first direction and no conductive fibres oriented in the first direction; and cross fibres oriented to cross the conductive fibres and the non-conductive fibres; electrically coupling the intermediate layer to the adjacent layers of conductive material so as to equipotentially bond the adjacent layers of the conductive material via the conductive portion of the intermediate layer; and curing the stack to mechanically bond the intermediate layer to the adjacent layers of the conductive material.

The method may further comprise infusing the stack with resin prior to the curing.

The wind turbine rotor blade spar cap manufactured by the method according to the third aspect of the invention may be the wind turbine rotor blade spar cap of the first aspect of the invention.

A fourth aspect of the invention provides a fibre fabric material for a wind turbine blade spar cap, the fibre fabric material having: a length in a first direction, a width perpendicular to the length, the width being shorter than the length, a first edge extending in the first direction, a conductive portion having conductive fibres oriented in the first direction, a first border portion between the first edge and the conductive portion, the first border portion having a plurality of non-conductive fibres oriented in the first direction and no conductive fibres oriented in the first direction, and cross fibres oriented to cross the conductive fibres and the non-conductive fibres.

The fibre fabric material of the fourth aspect may be particularly suitable for mechanically and equipotentially bonding adjacent strips of conductive material and may have the advantages described above with reference to the first aspect.

The fibre fabric material of the fourth aspect may optionally have properties substantially similar to those optional properties described above with reference to the fibre fabric material of the wind turbine rotor blade spar cap of the first aspect.

In this specification, terms such as leading edge, trailing edge, pressure surface, suction surface, thickness, and chord are used. While these terms are well known and understood to a person skilled in the art, definitions are given below for the avoidance of doubt.

The term leading edge is used to refer to an edge of the blade which will be at the front of the blade as the blade rotates in the normal rotation direction of the wind turbine rotor.

The term trailing edge is used to refer to an edge of a wind turbine blade which will be at the back of the blade as the blade rotates in the normal rotation direction of the wind turbine rotor.

The chord of a blade is the straight line distance from the leading edge to the trailing edge in a given cross section perpendicular to the blade spanwise direction.

A pressure surface (or windward surface) of a wind turbine blade is a surface between the leading edge and the trailing edge, which, when in use, has a higher pressure than a suction surface of the blade.

A suction surface (or leeward surface) of a wind turbine blade is a surface between the leading edge and the trailing edge, which will have a lower pressure acting upon it than that of a pressure surface, when in use.

The thickness of a wind turbine blade is measured perpendicularly to the chord of the blade and is the greatest distance between the pressure surface and the suction surface in a given cross section perpendicular to the blade spanwise direction.

The term spanwise is used to refer to a direction from a root end of a wind turbine blade to a tip end of the blade, or vice versa. When a wind turbine blade is mounted on a wind turbine hub, the spanwise and radial directions will be substantially the same.

The term fibre used in this specification is intended to refer to a bundle of filaments and may refer to a part also known as a yarn, roving, tow or strand.

The term edge is used in this specification to refer to a part of a material. It is noted that the edge of the material is the physical limit of the fibres oriented in the direction of the edge. Fringes of the material, which may comprise fibres extending through and beyond the edge, may extend beyond the edge of the material. Put another way, an edge of a material oriented in a first direction may be defined by an outermost fibre of the material oriented in the first direction.

<FIG> shows a wind turbine <NUM> including a tower <NUM> mounted on a foundation and a nacelle <NUM> disposed at the apex of the tower <NUM>. The wind turbine <NUM> depicted here is an onshore wind turbine such that the foundation is embedded in the ground, but the wind turbine <NUM> could be an offshore installation in which case the foundation would be provided by a suitable marine platform.

A rotor <NUM> is operatively coupled via a gearbox to a generator (not shown) housed inside the nacelle <NUM>. The rotor <NUM> includes a central hub <NUM> and a plurality of rotor blades <NUM>, which project outwardly from the central hub <NUM>. It will be noted that the wind turbine <NUM> is the common type of horizontal axis wind turbine (HAWT) such that the rotor <NUM> is mounted at the nacelle <NUM> to rotate about a substantially horizontal axis defined at the centre at the hub <NUM>. While the example shown in <FIG> has three blades, it will be realised by the skilled person that other numbers of blades are possible.

When wind blows against the wind turbine <NUM>, the blades <NUM> generate a lift force which causes the rotor <NUM> to rotate, which in turn causes the generator within the nacelle <NUM> to generate electrical energy.

<FIG> illustrates one of the wind turbine blades <NUM> for use in such a wind turbine. Each of the blades <NUM> has a root end <NUM> proximal to the hub <NUM> and a tip end <NUM> distal from the hub <NUM>. The blade <NUM> is arranged to extend away from the hub <NUM> in a spanwise direction S. A leading edge <NUM> and a trailing edge <NUM> extend between the root end <NUM> and tip end <NUM>, and each of the blades <NUM> has a respective aerodynamic high pressure surface <NUM> (i.e. the pressure surface) and an aerodynamic low pressure surface (i.e. the suction surface) <NUM> extending between the leading and trailing edges of the blade <NUM>.

Each blade has a cross section which is substantially circular near the root end <NUM>, because the blade near the root must have sufficient structural strength to support the blade outboard of that section and to transfer loads into the hub <NUM>. The blade <NUM> transitions from a circular profile to an aerofoil profile moving from the root end <NUM> of the blade towards the tip end <NUM>. The blade may have a "shoulder", which is the widest part of the blade where the blade has its maximum chord. The blade <NUM> has an aerofoil profile of progressively decreasing thickness towards the tip end <NUM>.

As shown in <FIG>, which is a cross sectional view of the blade <NUM> taken along the line A-A, the wind turbine blade <NUM> includes an outer blade shell formed of an upper part <NUM> and a lower part <NUM>, which together define a hollow interior space <NUM> with a shear web <NUM> extending internally between the upper and lower parts of the blade shell <NUM>, <NUM>. The blade shell parts may be two half-shells <NUM>, <NUM> which are separately moulded before being joined together (at the leading edge <NUM> and the trailing edge <NUM>) to form the blade <NUM>. It will be appreciated that the blade shell <NUM>, <NUM> need not be formed as two half-shells which are subsequently joined together but may be formed as a unitary shell structure, together with the shear web <NUM>, in a "one shot" single shell process. The blade shell may include a laminate composite material such as glass fibre and/or carbon fibre for example.

<FIG> shows a detail view of the region B, where the shear web <NUM> meets the blade shell <NUM>. A spar cap <NUM> may be incorporated into the outer shell <NUM>, as shown in <FIG>, or may be attached to the outer shell <NUM>. The spar cap <NUM> is an elongate reinforcing structure and may extend substantially along the full spanwise length of the blade <NUM> from the root end <NUM> to the tip end <NUM>. The spar cap <NUM> includes conductive material, such as carbon fibres. For example, the spar cap may include pultruded fibrous strips of material such as pultruded carbon fibre composite material or other carbon fibre reinforced plastic material.

The spar cap <NUM> may include a stack of layers of the conductive material. The shear web <NUM> may be adhesively bonded to an inner surface of the spar cap <NUM>. An outer surface of the spar cap <NUM> may sit adjacent a lightning conductor <NUM> in the outer surface of the blade shell <NUM>. As shown in <FIG>, the lightning conductor may be in the form of the metal foil <NUM> which may be separated from the outer surface of the spar cap <NUM> by one or more layers of insulating material <NUM>, such as glass fibre reinforced plastic. One or more further layers of glass fibre reinforced plastic may be provided over the outside of the metallic foil <NUM>. The layers collectively form an outer skin <NUM> of the blade shell <NUM>. One or more further layers of glass fibre reinforced plastic provide an inner skin <NUM> of the blade shell <NUM> with a core material between the outer skin <NUM> and the inner skin <NUM>. The core material may be a light structural foam, though other core materials such as wood, particularly balsa wood, and honeycomb may alternatively be used to provide a lightweight core material. It will be appreciated that a near identical connection may be made between the shear web <NUM> and the other side of the blade shell <NUM>.

The blade materials are laid up in a wind turbine blade shell mould, where they are then infused with resin to bond the blade materials together. As is well known in the art, the blade materials are covered with a sealed vacuum bag which is evacuated, and then resin is infused into the blade materials. The resin is then cured which may be at an elevated temperature. This is known as a vacuum assisted resin transfer moulding (VARTM) process.

<FIG> shows a cross-sectional view of the spar cap <NUM> before resin infusion, the cross-sectional view being taken such that it is viewed along a spanwise direction. The spar cap <NUM> has alternating layers of conductive material <NUM>, which may be pultruded carbon fibre strips, interspersed with intermediate layers <NUM> of a fibre fabric material. The pultruded carbon fibre strips <NUM> may provide structural strength to the spar cap <NUM>. However, it may be difficult for resin to infuse between the strips during the infusion process. The intermediate fibre fabric layers <NUM> establish a defined gap between the strips <NUM> so that resin can infuse between the strips. The intermediate layers <NUM> therefore act as an infusion promoting layer between the strips. The pultruded strips <NUM> are then bonded together so that the spar cap <NUM> may form a unitary structural part. The intermediate layers <NUM> help to ensure proper adhesion between the pultruded strips during infusion.

The pultruded strips <NUM> may be arranged in stacks, and as shown in <FIG>, the spar cap may be formed of two or more neighbouring stacks of strips. This may be advantageous when a curved spar cap is desired.

The intermediate layers <NUM> may be wider than the pultruded carbon fibre strips <NUM> in a chordwise direction. The edges of the intermediate layers <NUM> may therefore be spaced from the pultruded strips <NUM> when the intermediate layers <NUM> are laid up. This accommodates for misalignments when the intermediate layers <NUM> are placed on the carbon fibre strips <NUM>.

After curing of the spar cap <NUM>, a cured spar cap may be formed.

Due to the conductive nature of carbon fibre, electrical potential differences must be avoided between the pultruded carbon fibre strips <NUM>. Potential differences may be disadvantageous as they may lead to arcing within the spar cap <NUM>, which may damage the spar cap. It is therefore desirable that the intermediate layers <NUM> are electrically conductive in a through-thickness direction such that the carbon fibre pultruded strips <NUM> may be equipotentially bonded and so potential differences may be avoided.

<FIG> shows a schematic planform view of a known fibre fabric material <NUM> prior to resin infusion. The fibre fabric material comprises carbon fibre strands <NUM> oriented in both a first direction S, which may be a spanwise direction when the fibre fabric material is within to a wind turbine blade spar cap, and a crosswise direction perpendicular to the first direction. As is illustrated in <FIG>, the fibre fabric material <NUM> may extend further in a spanwise direction S than in a perpendicular, chordwise direction. The fibre fabric material <NUM> may therefore have a length in the first, spanwise direction S that is longer than a width in a perpendicular, chordwise direction.

The edges E of the fabric material <NUM> can be seen in <FIG>. The edges E are defined by the outermost fibres <NUM> oriented in the first direction S and fringes formed by the ends of the crosswise fibres <NUM> protrude further than the edges.

<FIG> shows the fibre fabric material <NUM> after resin infusion, where an infused fibre fabric material <NUM> is formed. It will be understood that the infused fibre fabric material <NUM> may be within a wind turbine blade spar cap and that therefore <FIG> may be considered as a cutaway view of a wind turbine blade spar cap.

The infused fibre fabric material <NUM> has carbon fibres <NUM> as described with reference to <FIG> and a matrix material <NUM>, such as an epoxy resin, which may be applied to the fibre fabric material via resin infusion. However, due to the nature of the fabric material, during the infusion process a fibre may slip off from the adjacent fibres and may become a loose fibre 202a, as shown in <FIG>. Where the loose fibre 202a is a conductive fibre, this may be disadvantageous as the loose fibre 202a may electrically couple the fibre fabric material to an adjacent structure, such as a heating element for de-icing the blade, or any other electrical sensitive component. Further, even where there is no electrical coupling to an adjacent component formed, the unpredictable nature of loose fibres may allow arcing to occur through a matrix material such that the loose fibre 202a may arc to itself or to the bulk of the fibre fabric material <NUM> during a lightning strike.

Commonly, when loose fibres such as the loose fibre 202a are identified, the fibre must be removed by cutting out the portion of the matrix material having the loose fibre and may require a patch repair on the blade.

In order to alleviate issues resulting from loose fibres, the present inventors have provided an intermediate layer <NUM> in the form of a new fibre fabric material <NUM>, shown in <FIG>. The fibre fabric material <NUM> has conductive fibres <NUM>, which may be carbon fibres, oriented in a first direction S, which may be a spanwise direction when the fibre fabric material <NUM> is laid up within a wind turbine blade spar cap, the conductive fibres <NUM> being arranged adjacent to each other across a conductive portion C of the material <NUM>. The material <NUM> also has border portions B adjacent the edges of the fibre fabric material <NUM>, arranged between the conductive fibres <NUM> oriented in the first direction S and the edges E of the material <NUM>. The border portions B contain non-conductive fibres <NUM> (shown as dashed lines) oriented in the first direction S and the border portions contain no conductive fibres.

The edges E are free of conductive fibres. As described above, the edges E, oriented in the first direction S, are defined by the outermost fibres oriented in the first direction S, which are non-conductive fibres <NUM>. Fringes formed of the cross fibres <NUM> may protrude beyond the edges E.

The fibre fabric material <NUM> also has cross fibres <NUM> oriented perpendicular to the first direction S. The cross fibres <NUM> may be non-conductive fibres, such as glass fibres, and the non-conductive nature of the cross fibres <NUM> may also mean that, in the case that a cross-fibre <NUM> is pulled out to become a loose fibre, the loose fibre is not a conductive fibre and so an undesirable electrical connection should not be formed.

The fibre fabric material <NUM> may be woven or stitched. A stitched material may be advantageous as it may lie flatter than a woven material, improving the structure of the stacks of layers within the spar cap <NUM>. However, a woven material may be advantageous as the woven nature of the conductive fibres <NUM> may improve conductivity through the thickness of the fibre fabric material <NUM>.

<FIG> shows an infused fibre fabric material <NUM>, where a matrix material <NUM> has been infused through the fibre fabric material. The resin infusion process may be substantially similar to the processes described above with reference to <FIG>. In this case, a loose thread 404a has formed during the resin infusion. However, since the loose thread 404a is non-conductive, the prospect of an undesirable electrical connection being formed is reduced.

The widths of the first border portions B (which are the border portions at the two edges of the material oriented in the spanwise direction, the border portions having widths in the chordwise direction, perpendicular to the spanwise direction), may be at least <NUM> millimetres. This may be advantageous where multiple loose threads may be formed from one side of the material. For this reason, the border portions may each comprise a plurality of non-conductive fibres oriented in the first direction, optionally at least <NUM> non-conductive fibres oriented in the first direction. By providing wider border portions having higher numbers of non-conductive threads, the prospect of undesirable electrical connections being formed by loose threads is further reduced.

By having non-conductive border portions adjacent to both edges E, the fabric material <NUM> may be agnostic to the way up in which it is oriented as it is laid and undesirable electric connections from either edge may be avoided.

The fibre fabric material <NUM> may have at least <NUM>, optionally at least <NUM>, conductive fibres oriented in the first direction within the conductive portion C. This may provide a high level of conductivity through the thickness of the material, to ensure equipotential bonding between adjacent carbon fibre pultruded strips. The conductive fibres may therefore extend across a width of at least <NUM> millimetres, preferably at least <NUM> millimetres.

Claim 1:
A wind turbine rotor blade spar cap (<NUM>), the spar cap having a length and comprising:
a stack comprising a plurality of layers of conductive material (<NUM>) and at least one intermediate layer (<NUM>),
wherein the layers of conductive material (<NUM>) each have a length along the length of the spar cap (<NUM>) in a first direction,
wherein the intermediate layer (<NUM>) is arranged between adjacent layers of the conductive material (<NUM>),
wherein the intermediate layer (<NUM>) includes a fibre fabric material (<NUM>) having:
a first edge (E) extending in the first direction,
a conductive portion (C) having conductive fibres (<NUM>) oriented in the first direction,
a first border portion (B) between the first edge (E) and the conductive portion (C), the first border portion having a plurality of non-conductive fibres (<NUM>) oriented in the first direction and no conductive fibres oriented in the first direction, and
cross fibres (<NUM>) oriented to cross the conductive fibres (<NUM>) and the non-conductive fibres (<NUM>), and
wherein the intermediate layer (<NUM>) is bonded with the adjacent layers of the conductive material (<NUM>) and is electrically coupled to the adjacent layers of conductive material so as to equipotentially bond the adjacent layers of the conductive material via the conductive portion (C) of the intermediate layer.