Technical field The present invention relates a wind turbine blade incorporating a component that can be fitted to the blade after the blade has been fabricated.

Background Wind turbines are vulnerable to being struck by lightning; sometimes on the tower, nacelle and the rotor hub, but most commonly on the blades of the turbine. A lightning strike event has the potential to cause physical damage to the turbine blades and also electrical damage to the internal control systems of the wind turbine. Wind turbines are often installed in wide open spaces which makes lightning strikes a common occurrence. Accordingly, in recent years much effort has been made by wind turbine manufacturers to design wind turbines so that they are able to manage effectively the energy imparted to them during a lightning strike in order to avoid damage to the blade and the associated cost of turbine down-time during blade replacement. Lightning protection systems for wind turbine blades are known. In one example, an electrically conductive lightning receptor element is arranged on an outer surface of the blade to receive a lighting strike. Since the receptor element is electrically conductive, lightning is more likely to attach to the receptor element in preference to the relatively non-conductive material of the blade. The receptor element is connected to a cable or 'down conductor' that extends inside the blade to the root and from there connects via an armature arrangement to a charge transfer route in the hub, nacelle and tower to a ground potential. Such a lightning protection system therefore allows lightning to be channelled from the blade to a ground potential safely, thereby minimising the risk of damage. However, the discrete receptor elements are relatively complex to install during fabrication of the blade and, moreover, they leave a significant portion of blade area exposed to a risk of lightning strike.

Observation of the effects of lightning strikes on turbine blades has revealed that the highest proportion of lightning strikes happen at the blade tips. To address this, WO2005/031 158 proposes a turbine blade having a solid metal tip. Configuring the tip of the blade in this way makes the tip highly resilient to frequent highly energetic lightning strikes and means that inspection and servicing events are required less often. However, one drawback of such a configuration is the challenge of integrating the metal tip into the rest of the blade structure in a seamless a way as possible. It is against this context that the invention has been devised.

Summary of the invention

Accordingly, the invention provides a wind turbine blade comprising a first blade portion connected to a second blade portion so as to define a junction between them that extends transversely around the blade surface, and a cowl structure fixed to the wind turbine blade and arranged to cover at least a portion of the junction, wherein the first blade portion is a blade tip module and has a metallic outer surface. An advantage of the invention is that the cowl structure protects the relatively vulnerable junction or joint between the two portions of a two-part wind turbine blade. The protection provided is against environmental attack which may otherwise damage the junction and potentially permit water to ingress into the interior of the blade, but also against lightning strikes attaching to possible sharp edges of the blade portions that are more exposed at the junction.

The blade tip module embodies a lightning receptor including a metallic outer surface. In order to provide a very robust solution for lightning capture, the metallic outer surface may be provided by the blade tip module being substantially of metal construction.

The cowl structure may be configured so that it covers parts of the junction that are considered to be most vulnerable to damage. However, in one embodiment the cowl structure covers substantially the entire length of the junction and so provides protection right the way around the blade surface.

In order to minimise the aerodynamic impact of the cowl structure it may be shaped to define an aerofoil profile in cross-section that is dimensioned so as to correspond to the cross section profile of the wind turbine blade at the junction. This ensures that the efficiency of the blade is not compromised by the attachment of the cowl structure to the blade. Although the cowl structure may be a multi-part component, in one embodiment it is a single-piece component without any split lines. This is convenient in that the cowl structure can simply be received over the blade tip and then manoeuvred into the appropriate spanwise position on the blade.

To enhance the electrostatic protection, the cowl structure may include a dielectric outer surface. This may be by way of a dielectric outer coating, for example a deposited polyimide thermoset coating, or by fabricating substantially all of the cowl structure from a suitable material, for example a polymeric material.

In an aerodynamic enhancement of the cowl structure, it may be provided with an upstanding fin or fence on an exterior surface that combats spanwise airflow along the blade in the region of the cowl structure. The fin may be configured so that it extends about the exterior surface of the cowl structure. In one alternative, however, it may extend just between the leading and trailing edges of the cowl structure.

To assist installers in mounting the cowl structure onto the blade, an inward facing surface of the cowl structure may includes a locating feature configured to locate the cowl structure at the junction of the blade. The locating feature may be in the form of a rib or pip to cooperate with a cooperating feature such as groove or depression at the junction of the blade. In a second aspect the invention applies to a cowl structure for a wind turbine blade, the cowl structure being configured to be applied to or received over a portion of the outer surface of the blade so as to protect a region of said blade. Preferred and/or optional features of the first aspect of the invention may be applied as necessary to the second aspect.

Although the invention has been described in relation to a wind turbine blade, it will be appreciated that the invention applies to other blades.

Brief description of the drawings For a more complete understanding of the invention, some embodiments will now be described with reference to the following drawings, in which:

Figure 1 is a front view of a contemporary three-bladed wind turbine;

Figure 2 is a perspective view of a single blade of the wind turbine of Figure 1 ;

Figure 3 is a view of the blade in Figure 2 but the blade is shown in a partly disassembled state;

Figure 4 is a perspective view of part of the blade in Figure 3, but is shown as including a cowl structure for protecting a region of the blade;

Figures 5 and 6 are perspective and side views, respectively, of the cowl structure carried by the blade in Figure 4;

Figure 7 is a section view along the line A-A in Figure 4;

Figure 8 is a perspective view of an alternative cowl structure to that shown in Figure 5; and

Figures 9a and 9b show alternative embodiments of cowl structures.

Detailed description

With reference to Figure 1 , a wind turbine 2 includes a tower 4 mounted on a ground platform 6 and a nacelle 8 is carried at the top of the tower 4. The nacelle supports a rotor assembly 10 that comprises three blades 12 mounted to a hub 14. The three- bladed 'horizontal axis' wind turbine configuration shown here is a common type of contemporary wind turbine, although it should be noted that other types are known, for example turbines having more or fewer blades, and also turbines in which the rotor assembly rotates on a vertical axis.

One of the blades 12 of the wind turbine 2 is shown in Figures 2 and 3: in Figure 2 the blade 12 is shown fully assembled and in Figure 3 the blade 12 is shown in a partially disassembled condition so that its principal components can be seen clearly. The blade 12 comprises a root 15, a tip 16, a leading edge 18 and a trailing edge 20, those edges corresponding to the direction of movement of the blade 12 through the air in use. The blade 12 also includes an aerodynamic surface 17 extending between the leading and trailing edges comprising a first, upper, surface 22 and a second, lower surface 24, although only the upper surface 22 is shown in the views of the blade 12 in Figures 2 and 3.

The blade 12 incorporates a lightning protection system 30 for attracting and managing lightning strikes. The lightning protection system 30 comprises a tip receptor 32 and down conductor cable 34 that is connected to and extends from the tip receptor 32 down through the interior or the blade 12 to the root 15 at which point it connects to a suitable grounding system 33. It should be noted that the components of the lightning protection system 30 are provided here to put the invention into proper context and should not be considered limiting on the invention; indeed, the lightning protection system 30 may incorporate other energy management devices such as surface protection layers, mid- blade bolt receptors and the like, although none are shown here for the sake of clarity.

In this embodiment, the tip receptor 32 has a metallic surface since it is substantially of metal construction thereby providing a metallic module at the end of the blade 12 at the point where lightning strikes are most common. During assembly of the blade 12, the tip receptor 32 is a separate component and is incorporated into the structure of the blade 12 to become an integral part of it. As such, the tip receptor 32 can be considered to be a 'first blade portion' and the remaining portion of the blade towards the root 15 can be considered to be a second or 'main blade portion' 35.

In Figure 3, the tip receptor 32 is shown as including connecting means in the form of a series of connecting rods 36, preferably being non-conductive such as pultruded glass- fibre rods, that are received in respective sockets (not shown) provided in the main blade portion 35. A suitable retaining mechanism is provided (not shown) to secure the connecting rods 36 in the sockets, in a manner described fully in WO2005/031 158, the contents of which are incorporated herein by reference. Note that this is one way in which the tip receptor 32 may be secured to the blade 12 but the skilled person will understand that there are numerous other ways to join these two components together. An attachment point 38 at the tip receptor 32 is also provided for fixing to the down conducting cable 34. The main blade portion 35 and the tip receptor 32 define a junction 40 at the interface between them. The junction 40 extends in a chord-wise direction around the perimeter of the blade 12, the chord-wise direction indicated here generally by the letter 'C. Parts of the junction 40 can be seen in more detail in Figures 4 and 7.

As can be seen, joining the tip receptor 32 to the main blade portion 35 results in a discontinuous surface 'S' either side of the junction 40 which is shown here as defining a groove 42. The discontinuity generally arises due to tolerance stacks of the components in the blade layup during fabrication affecting the accuracy of the interface between the tip receptor 32 and the main blade portion 35. A result of this is that sharp edges at the joining face of the tip receptor 32 are exposed which can encourage direct lighting strike attachment in this region. Lightning attachment in this region may cause damage to the main blade portion 35 and so measures to guard against such an occurrence are desirable.

Sometimes it is beneficial to insert a filling material in the groove 42 so that a smooth transition is provided between the main blade portion 35 and the tip receptor 32. However, this is a manual process which is time consuming and may require further finishing to hone the filling material to a suitably smooth surface and, even then, blade flex during use is likely to cause the filled joint to degrade and crack. At best, this reduces the aesthetic finish of the blade but, more seriously, it increases the vulnerability of the blade to environmental attack which may expose the edge of the tip receptor 32 given enough time.

With the above issues in mind, Figure 4 shows an embodiment of the invention that includes a cowl structure 50 which serves to protect the surface of the blade 12 in the region of the junction 40 between the tip module 32 and the main blade portion 35. The cowl structure 50 is received over the blade 12 and is dimensioned so as to locate onto the blade 12 in a spanwise position so as to cover the junction 40.

In the embodiment shown in Figure 4, the cowl structure 50 extends around the outer surface of the blade 12 like a band and covers the entirety of the junction 40 on both first and second aerodynamic surfaces of the blade. However, in other embodiments, the cowl structure 50 may be configured to cover those portions of the junction 40 that have been identified as being most vulnerable to damage. As can be seen particularly in Figure 5 and 6, the cowl structure 50 is shaped as an aerofoil in cross section profile having a leading edge 52, trailing edge 54 and upper and lower surfaces 56,58 extending between the leading and trailing edges 52,54. The cowl structure 50 is hollow so as to define an inside surface 51 and is sized so as to fit over the blade at a particular spanwise station. It will be appreciated therefore that the profile of the cowl structure 50 shown here is for illustration purposes only and, in practice, the cowl structure will be shaped to correspond to the blade profile. Note also that although the cowl structure 50 is shown here as extending across the blade in a direction that is perpendicular to the longitudinal axis of the blade, this direction is not essential and the cowl structure may extend in a direction that is generally transverse to, but not necessarily normal to, the blade axis, and so may extend obliquely to the blade axis.

Appropriate sizing of the cowl structure 50 determines the position at which it fits onto the blade 12. Although not shown here, the cowl structure 50 may be secured in place by suitable mechanical fasteners such as bolts. Mechanical fasteners provide a particularly strong connection between the cowl structure 50 and the blade 12 but, alternatively, the cowl structure 50 may be secured in place by adhesive bonding between the inner surface of the cowl structure 50 and the outer surface of the blade 12. The advantage of adhesive bonding is that the surface of the blade 12 does not have to be compromised by the installation of mechanical fasteners.

In this embodiment the cowl structure 50 is relatively rigid and is formed by way of a polyurethane injection moulding process so as to result in a single-piece unit that has no split lines. As a result, it is not necessary to use a joining technique to form the complete aerofoil section. However, it should be appreciated that it is not essential for the cowl structure to be a single-piece construction. For example, a single split line may be provided which would give the cowl structure 50 a 'clam-shell' like configuration. As such, it could be Opened' in order for it to be mounted onto the blade 12 and then closed so that it could be secured into position on the blade 12. Also envisaged are other embodiments in which the cowl structure 50 may be formed from two halves by virtue of split lines being formed on or near to the respective leading and trailing edges of the cowl structure 50. Note that polyurethane is given here as an example and should not be considered limiting. Other materials such as wood and ceramic may be used. Also, it is not essential that the cowl structure be formed as a rigid component; it could also be configured to have some resilience, for example a 'rubber' type polymeric material. In this embodiment however, as has been mentioned, the cowl structure 50 is a single- piece moulded polymeric component which makes for a relatively simple installation procedure whereby the cowl can be slid over the end of the blade 12 and moved into position. As an optional tactile aid to help locate the cowl structure 50, the inside surface 51 of it is provided with a locating means that is arranged to cooperate with the groove 42 on the blade surface. Here the locating means is depicted as a rib 60 that extends around the inside surface 51 of the cowl structure 50 in a longitudinal direction, that is to say between the leading and trailing edges 52,54. However, the locating means could take other forms, such as a series of discrete projections extending in a line.

In order to minimise its influence on the aerodynamics of the blade 12, it is preferred that the cowl structure 50 is relatively thin. However, to an extent the thickness of the cowl structure 50 is determined by the need for it to be quite rigid and also for it to provide a useful degree of protection to the blade 12. For example, the polymeric construction of the cowl structure 50 means that it is a dielectric and so provides a degree of protection from lightning strikes in the region covered by it. A different dielectric surface may be provided via a film deposition technique - e.g. polyimide film. This is useful since the presence of the cowl structure 50 supresses lightning leaders in this area in preference to the solid metal tip receptor 32 ensuring that lightning strikes are less likely to occur near to the junction 40 and more likely to attach to the tip receptor 32. In practice, it is envisaged that the thickness of the cowl structure 50, as indicated as T on Figure 7, may be in the range of 0.1 mm to 10mm, thereby extending from a 'thin film' to a reasonably thick cover.

In terms of its width, indicated as 'W on Figure 5, the cowl structure 50 must be wide enough to cover the junction 40 sufficiently, so it may be as narrow as around 10-20mm. The cowl structure 50 may be wider in circumstances where the junction 40 extends at an angle to the blade axis, as illustrated in Figure 9a. Here, the junction 40 extends at an angle to the blade axis 'L' and the cowl structure 50 is wide enough so as to encompass the entire junction 40.

The cowl structure 50 may be configured so that when mounted to the blade, its edges extend at an angle to the blade axis, as is shown in Figure 9b. Such a configuration may be particularly suitable to align the edges of the cowl structure to the 'inflow' direction of incoming air, denoted here by the arrows identified as I, which has the affect of reducing turbulence around the cowl structure 50.

An enhancement to the cowl structure 50 is shown in Figure 8. In this embodiment the cowl structure 50 provides the same protective benefits to the blade 12 but is also provided with a further aerodynamic surface in the form of a fin or 'fence' 62 which reduces the formation of spanwise airflow in the region of the blade tip 16. This benefits the efficiency of the blade 12 since spanwise airflow represents a loss of blade lift and, therefore, a loss of power being extracted from the wind.

The fence 62 is positioned approximately mid-way along the width W of the cowl structure 50 and extends along the upper surface 56 between the leading edge 52 and the trailing edge 54. In this embodiment, the fence 62 extends almost end-to-end along the upper surface of the cowl structure although this is not essential. Similarly, the central positioning of the fence 62 is not essential. The fence 62 may also be configured so as to be aligned with the inflow direction of air when the cowl structure 50 is mounted to the blade.

As can be seen in Figure 8, the fence 62 is shaped so that it is relatively tall near to the leading edge 52 of the cowl structure 50 but then tapers gradually towards the trailing edge 54. A fence 62 having a tapered shape is currently preferred since it has a drag reduction benefit and is achieved quite easily by the moulding process. However, different fence shapes are possible, which may be easier to fabricate if different methods and materials for constructing the cowl structure are chosen. For example, the fence 62 may be uniform in height between its ends.

In this embodiment a single fence 62 is provided on the upper surface 56 of the cowl structure 50. However, it should be appreciated that a fence 62 may be provided on both upper and lower surfaces 56,58 of the cowl structure. In such circumstances, the fences 62 may be configured so that they are aligned at different angles with respect to the blade axis L.

Some variations on the specific embodiments shown in the figures have already been described. However, the skilled person would understand that various modifications could be made to those specific embodiments without departing from the inventive concept as defined by the claims. For example, the explanations about the thickness and width of the cowl structure 50 are given to provide a sense of scale to the specific embodiment and should not be considered limiting. The skilled person would therefore understand that the cowl structure 50 could be configured as necessary so as to fit any wind turbine blade as required.

Furthermore, the skilled person would understand that although the cowl structure 50 has been described as being moulded from polyurethane, this is only meant by way of example and other materials would be acceptable. For example, the cowl structure 50 could be made from other mouldable plastics, from composite materials (e.g. FRP, CFRP), or even from non-plastic products such as a flexible plywood and ceramic.