FIELD OF THE INVENTION

The present invention relates to a wind turbine blade and a method of constructing a wind turbine blade.

BACKGROUND OF THE INVENTION

There is a known problem for wind turbine blades that lightning may strike the blade and may thereby damage the blade. Wind turbine blades are therefore commonly employed with lightning protection systems arranged to conduct electricity from lightning through the blade without incurring damage. In some cases, there may be a lightning conductor through the length of a wind turbine blade, extending from the tip of the wind turbine blade to the wind turbine hub through the middle of the blade.

The lightning conductor may be coupled to the surface of the blade via one or more lightning receptors. However, such lightning receptors may interfere with the aerodynamics of the wind turbine blade. For example, the lightning receptor may be a bolt protruding above the head of the blade, which may increase drag of the blade.

Attempts have been made to decrease the aerodynamic effect of the lightning receptors. Such solutions involve providing a hole in the wind turbine blade with a counterbore in which the lightning receptor may sit, and the solution may also require machining the head of the lightning receptor to be flush with the wind turbine blade surface or shaping the lightning receptor to have a generally an aerofoil shape. However, these methods entail significant constructional difficulty as the machining of the head of the lightning receptor may be difficult without damaging the blade and may require high precision to within a small tolerance. Similar issues are found with counterbore a hole in a blade surface as this requires extra machining of the blade and has the possibility of causing unwanted damage. Attaching a shaped bolt to a wind turbine blade may also be difficult as specialised tools may be required. The present inventors have therefore sought to provide a wind turbine blade having a lightning receptor with reduced aerodynamic drag and reduced manufacturing difficulty.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a wind turbine blade comprising: a blade surface extending between a blade root and a blade tip and a between a blade leading edge and a blade trailing edge; a lightning receptor having a head disposed outwardly of the blade surface and a shaft extending through the blade surface; and a fairing fixed to the blade surface proximate the lightning receptor, the fairing being separate from the lightning receptor.

By providing a fairing separate from the lightning receptor, a standard bolt, such as a hex bolt or other shaped bolt having a polygonal cross-section, may be used as the lightning receptor and the separate fairing may be applied separately, allowing the bolts to be incorporated within a generally aerofoil shape such that aerodynamic drag due to the bolt may be reduced. The “fairing being separate from the lightning receptor” means that the fairing is a separate component to the lightning receptor.

The requirement for any counterboring of a hole in the wind turbine blade may also be avoided, as the lightning receptor may protrude above the wind turbine blade surface, with the fairing reducing drag which would otherwise be generated by the lightning receptor. The tolerances involved in attaching a fairing may also be more generous than those associated with counterboring a hole, and achieving the required tolerance may be easier. Similarly, machining of the lightning receptor may be avoided and the associated issues of providing a low tolerance and possible damage to the blade may be avoided. In the present disclosure, the term ‘counterboring’ also encompasses ‘countersinking’.

Hence, the lightning receptor may be more easily attached to the wind turbine blade and the overall arrangement may still have a low drag.

At least a portion of the fairing may be arranged between the lightning receptor and the blade trailing edge. The fairing may have a height in a direction extending away from the blade surface, and the height may be greater proximate the lightning receptor and may be lesser distal from the lightning receptor.

The fairing may be made of a different material from the lightning receptor, optionally the fairing is made from a less dense material than the lightning receptor.

The fairing may be made of a less electrically conductive material than the lightning receptor, preferably the fairing comprises an insulating material.

The fairing may have a length in a chordwise direction, wherein the head of the lightning receptor has a diameter, and wherein the length of the fairing may be between 3 and 5 times more than the diameter of the head of the lightning receptor.

The fairing may have a hole therethrough and the lightning receptor may extend through the hole.

The hole may comprise: a first hole section distal from the blade surface; a second hole section proximate the blade surface, narrower than the first hole section; and an interface surface between the first and second hole sections, the interface surface engaging the head of the lightning receptor. The fairing may be fixed to the blade surface by the lightning receptor.

The lightning receptor may be disposed in a recess at a first end of the fairing, the first end being an end of the fairing opposite to the trailing edge of the blade.

The fairing may not be in contact with the lightning receptor.

The fairing may comprise a first fairing portion and a second fairing portion, the first and second fairing portions being separable, the first fairing portion being disposed between the lightning receptor and the trailing edge and the second fairing portion being disposed between the lightning receptor and the leading edge. The lightning receptor may extend from the blade surface by a first distance and wherein the fairing has a height in a direction extending away from the blade surface, the height being equal to the first distance in a region adjacent the lightning receptor.

A plate may be provided between the head of the lightning receptor and the blade surface, the plate having a lower electrical conductivity than the lightning receptor.

A lightning conductor may be disposed inside the wind turbine blade, wherein the lightning receptor is electrically coupled to the lightning conductor.

A second aspect of the invention provides a method of constructing a wind turbine blade, the method comprising: providing a blade surface extending between a blade root and a blade tip and between a blade leading edge and a blade trailing edge; providing a lightning receptor and a fairing separate from the lightning receptor; fixing the lightning receptor to the blade surface such that a shaft of the lightning receptor extends through the blade surface and a head of the lightning receptor protrudes above the blade surface; and fixing the fairing to the blade surface proximate the lightning receptor.

By providing a separate fairing and lightning receptor, the method may provide a more simple way to provide a low drag lightning receptor on a wind turbine blade. In particular, the method may avoid the difficulties associated with counterboring a hole in a wind turbine blade surface as described above, may avoid the requirement to machine the lightning receptor, and may allow a lightning receptor to be used which may be attached to the wind turbine blade more easily. In this way the advantages of the second aspect may be substantially similar to those of the first aspect.

The method may further comprise placing the fairing between the blade surface and the lightning receptor before fixing the fairing to the blade surface, and wherein the fairing is fixed to the blade at the same time as the lightning receptor. Alternatively, the steps of fixing the lightning receptor to the blade and fixing the fairing to the blade may be performed separately. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 shows a wind turbine;

Figure 2 shows a wind turbine blade;

Figure 3a shows a general view of a fairing for a wind turbine blade;

Figure 3b shows a cross-sectional view of the fairing;

Figure 3c shows a plan view of the fairing;

Figures 4a to 4c show cross-section of alternative fairings for a wind turbine blade;

Figure 5a shows a general view of a further alternative fairing for wind turbine blade;

Figure 5b shows a cross-section of the further alternative fairing;

Figure 5c shows a plan view of the further alternative fairing;

Figure 6a shows a fairing applied to the surface of a wind turbine blade;

Figure 6b shows a cross-section of the fairing applied to the wind turbine blade; and

Figure 7a and 7b show cross-sections of alternative fairings applied to wind turbine blades.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 shows a wind turbine 10. The wind turbine comprises a tower 12 extending from a foundation 14 and supporting a nacelle 16. A rotor 18, having blades 20 extending radially outwardly is rotatably mounted on the nacelle 16. As wind acts on the blades 20, the rotor 18 will rotate and will transfer torque to a gearbox and to a generator within the nacelle 16. Accordingly, the wind turbine 10 may generate electrical energy from wind energy.

Figure 2 shows a general view of a wind turbine blade 20. The wind turbine blade 20 has a root end 22, which may be fixed to a hub of the rotor of the wind turbine and a tip end 24 which may be disposed away from the hub. A leading edge 26 and a trailing edge 28 extend between the root end 22 and tip end 24. The leading edge 26 is arranged to be at a front of the blade as the blade moves into the wind, with the trailing edge 28 at the back as the blade moves into the wind. The blade 20 also has two surfaces: a pressure surface 30 and a suction surface 32.

Within this specification, a spanwise direction of the blade 20 is intended to mean a direction from the root end 22 to the tip end 24, i.e. a radial direction of the rotor 18. A chordwise direction of the blade 20 is a direction from the leading edge 26 to the trailing edge 28, and is the direction of travel of the blade in normal use. A thickness direction of the blade 20 is perpendicular to the spanwise and chordwise directions and is a direction from the pressure surface 30 to the suction surface 32.

Figure 3a shows a general view of a fairing 100 (which is shown also in a side view in Figure 3b and in a plan view in Figure 3c), which may be applied to a wind turbine blade such as the wind turbine blade 20. The fairing 100 has an upper surface 102, a side surface 104 and a lower surface 108. The fairing 100 may also have a hole 106 extending through it. The fairing 100 may be placed around a lightning receptor applied to a wind turbine blade and may thereby present a generally aerodynamic shape to incident airflow, reducing the drag which would otherwise be generated by the bluff body shape of the lightning receptor. By arranging the lightning receptor within a hole 106 within the fairing 100, the lightning receptor may be surrounded by the fairing and the drag generated by the arrangement may thereby be further reduced. The fairing 100 may also provide improved protection to the surface of the wind turbine blade in this case, as set out below.

The fairing 100 may also act as an insulator to protect the wind turbine blade from lightning when the lightning receptor is struck by lightning. To this end, the fairing 100 may be formed from an insulative material, having a lower electrical conductivity than the lightning receptor. This may prevent electricity from arcing between the lightning receptor and the surface of the blade, reducing the chance of damage to the surface of the blade when lightning strikes the lightning receptor.

The fairing 100 may also be formed of a less dense material than the lightning receptor. For example, the fairing may be formed of a plastic or composite material. This may allow the arrangement to have a lower overall weight, consequently, the blade may need to support only a lesser weight, saving material as the blade strength may be reduced relative to that for a blade having a heavier fairing.

Figure 3b shows a cross-section of the fairing 100 taken along the line A-A shown in Figure 3a. In this example, the fairing 100 has a constant height H, which is a distance the fairing 100 extends away from the blade surface. The height H may be substantially similar to the distance a lightning receptor extends away from the blade, meaning that the frontal area of the fairing 100 may be minimised and the entirety of the frontal area of the lightning receptor may be covered by the fairing 100, so that drag may be reduced at an optimal level.

The hole 106 through the fairing 100 may have a diameter D. The diameter D may be substantially similar to a diameter of a head of a lightning receptor such that the head of the lightning receptor may fit tightly within the hole 106 and the airflow may be minimally disturbed by the gap between the fairing 100 and the lightning receptor within the hole 106. Alternatively, the hole 106 may be sized so that the fairing 100 does not contact the lightning receptor. The hole 106 may be spaced such that a tool, such as a spanner or socket, may be more easily applied to fix the lightning receptor to the blade. Thus, the assembly of the arrangement may be simplified.

The fairing 100 may also be applied to the blade before or after the lightning receptor, and more generally may be applied separately. This may allow more flexibility in the assembly process.

A length L of the fairing 100 in a chordwise direction of the blade may be greater than three times the diameter D and/or may be less than five times the diameter D. The diameter D of the hole 106 may determine the thickness of the aerofoil shape of the upper surface 102 of the fairing 100 (measured in a spanwise direction of the blade). The upper surface may therefore have an aerofoil shape, when viewed from above, having a thickness of between 33% and 20%. Such an aerofoil may have a sufficiently low drag while also having an acceptably small size and accordingly an acceptably low weight.

The generally aerofoil shape of the fairing 100 can be seen in both of Figures 3a and 3c. By providing an aerofoil shape having a relatively low thickness, the airflow around the lightning receptor may be substantially undisturbed. This may provide a low drag and a small wake. Consequently, the lift and drag of the wind turbine blade to which the lightning receptor may be applied may be substantially unaffected.

Figures 4a to 4c show alternative fairings 100. The fairing 100 shown in Figure 4a has a variable height across its length. The height is greater in a region proximate the hole 106 and may be substantially equal to the height of a bolt in this area. The height of the fairing 100 may then taper away so that it is thinner at the leading and trailing ends of the fairing. This may allow the fairing 100 to have an aerodynamic profile in both cross-sectional and plan views and may thereby further reduce the drag generated.

Figure 4b shows a cross-section of a fairing 100 where the attachment surface 108 of the fairing 100 is curved so as to mimic the curvature of the wind turbine blade to which it is to be fitted. In this case, the fairing 100 may be made of a substantially rigid material as it does not need to curve or bend in order to adapt to the shape of the wind turbine blade. Where the fairing 100 is attached to the wind turbine blade via adhesive, the shaping of the fairing 100 may also allow a stronger adhesive connection to be formed.

Figure 4c shows a fairing 100 wherein the hole 106 has a first hole portion 106a, which is further from the blade surface and adjacent the upper surface 102. The diameter of the first hole portion 106a may be substantially similar to the diameter of a head of the lightning receptor to be received in the hole 106. The hole 106 also has a second hole portion 106b adjacent the blade surface and the blade attachment surface 108. The second hole portion 106b may have a smaller cross-section than the first hole portion 106a and the diameter of the second hole portion 106b may be substantially similar to a diameter of a shaft or shank of a lightning receptor to be received in the hole 106. Between the first and second hole portions 106a, 106b, there is a first engagement surface 106c, which may be an annular surface arranged to engage with the head of the lightning receptor to be received in the hole 106 and which may thereby allow the lightning receptor to hold the fairing 100 onto a blade surface. The engagement surface 106c and the portion of the fairing between the engagement surface 106c and the wind turbine blade may also act to insulate the lightning receptor from the blade surface electrically. This arrangement may thereby allow the fairing 100 to be more securely held onto the wind turbine blade and the portion of the fairing 100 between the head of the lightning receptor and the blade surface may also act to protect the surface of the blade from lightning.

In this arrangement, the lightning receptor and the fairing may be fixed to the blade at the same time. Consequently, only a single fixing step may be required and so the assembly method may be faster.

It will be understood that those aspects of the fairings shown in Figures 4a, 4b and 4c may be combined with the fairing shown in Figures 3a to 3c or the fairings shown in Figures 5a to 5c (described below) as appropriate.

Figure 5a shows a further alternative fairing 100, in which the fairing is formed from two separable pieces. The fairing 100 has a first fairing portion 100a, which is arranged to be disposed between the lightning receptor and a leading edge of the wind turbine blade and a second fairing portion 100b, which is arranged to be disposed on the wind turbine blade surface between the lightning receptor and the trailing edge of the blade. The fairing 100 is divided into two separate pieces and may be applied separately from the lightning receptor and so may be connected to the wind turbine blade before or after the lightning receptor. Further, the fairing may offer a greater flexibility, and, due to the reduced length of each portion of the fairing, may more closely follow the curvature of the wind turbine blade surface. The two-part fairing may therefore be easier to assemble and may be easier to manufacture to a required tolerance due to the reduced length of each part.

So that the fairing portions 100a, 100b may fit closely with a lightning receptor, each fairing portion may have a recess 110 at an end thereof. The recesses 110 together may form a hole for receiving the lightning receptor. By providing recesses 110 which each partially surround the lightning receptor, the fairing portions 100a, 100b may be more easily arranged relative to the lightning receptor, allowing a closer fit between the fairing portions 100a, 100b and the lightning receptor. Consequently, drag may be further reduced.

Figures 5b and 5c show alternative views of the fairing 100 in a cross-section taken along line B-B and in plan view respectively.

From Figures 5b and 5c, it can be seen that the fairing may have a variable thickness similar to that of the fairing of Figure 4a. The fairing may also have a generally aerofoil shape when viewed from above, with the two parts 100a, 100b of the fairing and the lightning receptor being together forming an aerofoil shape. The lightning receptor may have a diameter greater than the width of the fairing such that a portion may be exposed as an outer surface of the aerofoil shape as viewed from above, or the fairing 100 may substantially surround the lightning receptor.

It will be understood that various features of the fairings shown in Figures 3a to 4c may be employed in the fairing shown in Figure 5a. The fairing may, for example, be a two- part fairing with a constant cross-sectional thickness or the recess may be staggered with two portions of differing area and a portion of an annular engagement surface for engaging with a head of a lightning receptor. The fairings may be flexible so that they can adapt to the curvature of the wind turbine blade surface or may be rigid with a predefined curvature.

In some examples, a fairing may be constructed which has only the second portion 100b of the fairing 100 shown in Figure 5a. In such cases, the lightning receptor may form a leading edge of a generally aerofoil shaped construction with the fairing, and the fairing may provide a trailing edge. In such examples, the lightning receptor may be received within a recess at an end of the fairing opposite to the trailing edge of the aerofoil. This may provide a further simplified construction while still reducing drag. By providing at least a portion of the fairing 100 between the lightning receptor and the trailing edge of the blade, the fairing 100 may reduce the wake generated by the lightning receptor, thereby reducing the drag which would be generated by the lightning receptor. Figure 6a shows a fairing 100 in situ on a wind turbine blade surface 30 and having a lightning receptor 150 extending therethrough. The wind turbine blade is also shown in cross-sections in Figure 6b, the cross-section being taken along the line C-C.

It can be seen that the lightning receptor 150 may have a head 152 and a shaft 154. The head 152 has a diameter D _H and the shaft 154 has a diameter D _s , the head diameter D _H being greater than the shaft diameter D _s . The head 152 is arranged to protrude above the wind turbine blade surface 30 so as to attract lightning strikes and, in the case shown in Figures 4c and 7b, the head 152 may act to secure the fairing 100 onto the blade surface 30.

The shaft 154 extends through the wind turbine blade surface 30 and may be threaded in order to secure the lightning receptor 152 to the wind turbine blade or may have a barbed shaped. Alternatively, the lightning receptor may be adhered to the wind turbine blade surface 30 and the shaft may not provide a fixing function. The shaft 154 may act to conduct electricity from the head 152 to an internal lightning conductor 170, which may be disposed inside the wind turbine blade. The internal lightning conductor may therefore safely conduct electricity from lightning to ground without damaging the wind turbine. The shaft 154 may therefore have a length L _s , which is sufficiently long to extend through a composite shell of the wind turbine blade and to fix the receptor 150 to the wind turbine blade. The head 152 may have a length L _H , which is sufficient to attract lightning to the lightning receptor 150 and to reduce the chance of a lightning strike hitting the surface of the wind turbine blade directly.

The height of the fairing H may be the same as the height of the head l_ _H in a region adjacent to the head, where the head 152 sits directly on the wind turbine blade surface. Where the head 152 abuts an engagement surface within the fairing, the height of the fairing may be equal to the distance the head 152 protrudes above the wind turbine blade surface, as shown in Figure 7b, which is described below.

Figures 7a and 7b show cross sections of alternative wind turbine blades. As shown in Figure 7a, the wind turbine blade may further comprise an insulative plate 180, which may be arranged between the fairing 100 and the wind turbine blade and between the head 152 of the lightning conductor and the wind turbine blade. The insulative plate 180 may help to prevent electricity from lightning strikes being transmitted into the surface 30 of the wind turbine blade. The chance of lightning damaging the wind turbine blade may therefore be further reduced.

The insulative plate 180 may also provide thermal insulation. In the event of a lightning strike, the head 152 can reach a high temperature, and the insulative plate 180 will prevent thermal damage to the blade structure.

Figure 7b shows an arrangement in which the lightning receptor 150, and in particularly the head 152 of the lightning receptor 150, may hold the fairing 100 onto the wind turbine blade surface 30 by engaging an engagement surface 106c of the fairing 100.

The faring 100 may therefore be the fairing shown in Figure 4c. In this case, the fairing 100 may also act to insulate the wind turbine blade from lightning strikes by providing an insulative material between the head 152 and the blade. Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.