PROTECTION SYSTEMS FOR WIND TURBINE BLADES

Technical field

The present invention relates to wind turbine blade structures and associated fabrication processes for improving the resilience of wind turbine blades to lightning strikes. The invention also extends to preformed components for use in such wind turbine blades, and methods for fabricating such preformed components.

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 cost associated with turbine down-time during blade replacement.

In general, 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 receptors 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. Such a receptor arrangement provides discrete conductive points to which lightning may attach. To increase the effectiveness of such a system, wind turbine blades may also have a conductive layer that is laid over the outer surface of the blade shell so as to make contact with the receptor elements. The conductive layer increases the area of the blade that can receive lightning, thereby increasing the rate at which the receptor elements can capture lightning strikes safely. Although a conductive layer used in this way can be said to increase the capability of the lightning protection system to intercept lightning strikes, such a system can be complex to manufacture since the conductive layer must be added to the blade after the blade shell has been fabricated. This requires an additional time-consuming manufacturing step thereby increasing assembly time and cost.

It is against this context that the invention has been devised. Summary of the invention

In one aspect, the invention may be defined as a preformed component of a lightning protection system for a wind turbine blade, the preformed component comprising a sheet of conductive material having an edge that defines a perimeter of the sheet, wherein an edge protection structure is attached to the sheet so as to envelop the edge along at least a portion of the perimeter.

The invention may also be expressed, in a second aspect, as method of forming a preformed component of a lightning protection system for a wind turbine blade, the method comprising providing a sheet of conductive material having an edge that provides a perimeter of the sheet and applying an edge protection structure to the edge so that the edge protection structure envelops the edge along at least a portion of the perimeter. The edge protection structure prevents the edge of the sheet of conductive material causing damage to adjacent components by covering up sharp pieces thereof.

In one embodiment, the edge protection structure envelops the edge so as to be in contact with a first surface and a second surface of the sheet. In effect, therefore, the edge protection structure 'wraps' around the edge and so covers up any sharp elements that project from the edge. A further benefit is that the thickness of conductive material is increased along the perimeter at the edge region which improves the current carrying capacity of the component.

In order to wrap around the edge, the edge protection structure may be configured to define a channel that receives the edge of the conductive sheet, whereas first and second wall portions of the structure contact the first and second surface of the sheet respectively.

In one embodiment, the edge protection structure is an elongate strip that is folded along a longitudinal axis thereof so as to define the channel and first and second wall portions. An enhancement may be provided in the form of inwardly direct tab portions formed on the wall portions. The tab portions serve to eliminate any sharp parts along the wall portions by ensuring they are contained within the interior of the folded strip. The invention can also be expressed, in a third aspect, as an edge protection structure for a surface protection layer in a lightning protection system, the edge protection structure comprising an elongate strip folded along its longitudinal axis so as to define a channel and first and second wall portions. The edge protection structure may be fixed to the edge by suitable techniques, such as mechanical fastening means, adhesive bonding, or by welding.

The conductive sheet may be a metallic foil which is lightweight, yet highly conductive. Further weight saving benefits may be achieved by the use of a mesh, or an expanded metal foil. The expanded metal foil confers a benefit in terms of its structural integrity. Similarly, the edge protection structure may be formed of a metallic foil, a mesh or an expanded metal foil.

In the case where the edge protection structure is an elongate strip folded along its longitudinal axis, it may be stored in coiled form on a storage reel from which it may be unreeled and then cut to an appropriate size. The invention may therefore also be expressed in a further aspect as a storage reel containing an edge protection structure as defined above. The invention also extends to a wind turbine blade incorporating a lightning protection system including a surface protection layer comprising a preformed component as defined above. It will be appreciated that preferred and/or optional features of the various aspects of the invention as explained above may be combined with each other as appropriate.

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

Figure 1 is a front view of a wind turbine in which the invention may be used;

Figure 2 is a top view of a blade of the wind turbine incorporating a lightning protection system;

Figure 3 is an enlarged view of the wind turbine blade in Figure 2 showing a surface protection layer of the lightning protection system in more detail;

Figure 4 is a perspective view showing an assembly sequence of a surface protection layer;

Figures 5 and 6 are views of a surface protection layer of an embodiment of the invention, wherein the layer in Figure 5 is only partially assembled;

Figures 7a, 7b and 7c illustrate how an embodiment of edge protection structure may be applied to the surface protection layer in Figure 5 and 6;

Figures 8a-c illustrate alternative forms of edge protection structures; and

Figure 9 illustrate a means to store an edge protection structure.

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 Figure 2 and incorporates a lightning protection system 20. The blade 2 is formed from a blade shell 22 having two half-shells. The half-shells are typically moulded mainly from glass-fibre reinforced plastic (known as 'GFRP' or, simply 'GRP') that comprises glass fibre fabric embedded in a cured resin matrix. The precise construction of the blade shell 22 is not central to the invention and so further detailed description is omitted for clarity.

The blade comprises a root end 24, at which the blade 2 attaches to the hub 14 of the wind turbine 2, a tip end 26, a leading edge 28 and a trailing edge 30. Upper and lower surfaces 32 of the blade 2 define an aerodynamic profile that extends between the leading edge 28 and the trailing edge 30. Only the upper surface is shown here.

Turning to the lightning protection system 20, in overview the main components are a tip receptor module 34, a surface protection layer 36 and a set of down conductors 38 that electrically link the tip receptor module 34 and the surface protection layer 36 to an energy transfer device 40 at the root end 24 of the blade which serves to transfer the energy imparted to the blade in a lightning strike event to suitable conduction systems located in the hub 14 as is known in the art. The set of down conductors 38 includes two such conductors running along the length of the blade 12 from the tip end 26 to the root end 24, generally being arranged adjacent the leading edge 28 and trailing edge 30 of the blade 12, respectively. Note that down conductors are known generally in the art.

It should be noted at this point that the components of the lightning protection system 20 are provided here to put the invention into proper context and should not be considered limiting on the invention; indeed, the lightning protection system 20 may incorporate other energy management devices such as mid-blade bolt receptors and the like, although none are shown here for the sake of clarity. In this embodiment, the tip receptor module 34 has a metallic surface since it is substantially of metal construction thereby providing a robust lightning strike point at the tip end of the blade 12 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 in a manner described fully in WO2005/031 158.

The capability of the lightning protection system 20 against lightning strikes is enhanced by the surface protection layer 36 which extends over a region of the blade surface 32 over approximately a third of the spanwise length of the blade, in this example.

As has been mentioned, the surface protection layer 20 comprises a conductive layer that is integrated into the surface of the blade 12. The surface protection layer 20 is shown here only on one side of the blade but it should be appreciated that it may be provided on both sides of the blade 12. The conductive layer may be a metallic screen or mesh, but preferably an expanded metal foil that protects the underlying blade elements from direct lightning strikes over a large area of the blade and which is connected to the down conducting system 36 by any suitable means, for example by conductive connectors that extend through the surface of the blade and connect to the down conductors.

The thickness of the layer 20 is such that the aerodynamic profile of the blade 12 is unaffected and so it is preferred that the layer 20 is less than 5mm in thickness. In principle, an expanded foil of any metallic material is acceptable as long as it provides the necessary current-carrying and charge dissipation capability, although aluminium and copper foils are currently preferred.

Figures 3 and 4 show the surface protection layer 20 in more detail, although not to scale. Here it can be seen that the surface protection layer 20 is connected to the down conductors 38 by a plurality of connectors 42. Four connectors 42 are shown in this view of the surface 32 of the blade 2, two being adjacent the leading edge 28 of the blade 12 and two being adjacent the trailing edge 30 of the blade 12.

In overview, and as has been mentioned, the surface protection layer 20 incorporates a conductive layer currently envisaged to be expanded aluminium foil. In Figure 4, the surface protection layer 20 is shown in exploded view for clarity against a portion of a blade mould 44. The surface protection layer 20 includes three main components: an outer insulating layer 50, an inner insulating layer 52 and a conductive layer 54 sandwiched between the insulating layers 50, 52. Note that the connectors 42 are not shown in Figure 3.

Both the outer insulating layer 50 and the inner insulating layer 52 are glass fibre fabric. The outer insulating layer 50 becomes the outer surface or skin of the blade 12 once the blade 12 is fully fabricated. The inner insulating layer 52 covers the conductive layer 54 and protects the rest of the blade layup from any sharp edges of the conductive layer.

During assembly of the blade 12, the outer insulating layer 50, the conductive layer 54 and the inner insulating layer 52 are built up on the mould surface 44 following which the remaining components of the blade, such as structural foam cores, functional components and the like, are applied on top of the conductive layer 54. However, manufacturing benefits may be achieved by forming the first and insulating layers 50,52 and the conductive layer 54 as a preformed component.

Although the surface protection layer 20 described with reference to Figure 3 has a benefit in terms of improving the ability of the lightning protection system to manage lightning strikes across a wide area of the blade surface, it also has disadvantages in that it is relatively expensive to fabricate due to the need for sandwiching the material between protective fabric layers and also that it problematic to handle to the sharp edges, particularly when it is in the form of an expanded metal foil. Embodiments of the invention that addresses these drawbacks will now be described with reference to Figures 5 to 9.

Turning firstly to Figure 5 and 6, there is shown a preformed component 60 that can be used as a surface protection layer in a lightning protection system in the context described above.

The preformed component 60 comprises a conductive layer or sheet 62 capable of receiving and carrying energy imparted to it by a lightning strike. It is currently envisaged that the conductive sheet 62 is an expanded metallic foil, preferably of aluminium, although this does not preclude the use of other expanded metal foils, such as copper. Aluminium is currently preferred chiefly due to its cost effectiveness and light weight. Moreover the conductive sheet 62 may be other types of metallic foils such as solid foils or a mesh.

The conductive sheet 62 comprises first and second surfaces 64,66 (upper and lower surfaces, respectively, are shown more clearly in Figures 7a-c) and an edge 68 that defines a perimeter 70 of the sheet 62. Since the conductive sheet 62 is generally rectangular in this embodiment there are four edge portions 68a-d.

As is particularly the case with expended metal foil, the perimeter 70 of the sheet 62 tends to be sharp due to the severing of the individual strands of the foil as it is cut to size. The sharpness along the edge of the sheet 62 can cause damage to blade components during the blade layup process and can also be hazardous to manufacturing personnel as the sheet 62 is handled in order to place it into the layup. To combat this, the preformed component 60 includes an edge protection structure 72 which wraps around, or envelops, the edge of the conductive sheet 62 and extends along at least a portion of the perimeter.

In Figure 5, four edge protection structures 72 are shown; one for each linear edge portion 68a-d of the generally rectangular conductive sheet 62. Here, the edge protection structures 72 have not been applied to the conductive sheet 62. However, Figure 6 shows the edge protection structures 72 positioned onto the conductive sheet 62.

Since the edge protection structures 72 envelop the edge of the conductive sheet 62, they cover and contain any sharp parts that exist along the associated part of the perimeter 70.

Figures 7a-c show the edge protection structure 72 in more detail. As can be seen, in this embodiment the edge protection structure 72 takes the form of an elongate strip 74 of material. The strip 74 is conductive and so may be metallic, and preferably in the form of a metal contiguous foil, mesh or expanded metal foil. Most preferably, the strip 74 is formed of expanded metal foil in common with the conductive sheet 62.

The elongate strip 74 is folded along its longitudinal axis 'L' and in this way defines a central channel 76 and first and second wall portions 78a,78b. When applied to the conductive sheet 62, the channel 76 receives the edge 68 of the sheet 62 whilst the first and second wall portions 78a,78b extend over the upper and lower surfaces 64,66 thereby enveloping the edge 68 within it.

In addition to the material handling benefit, the application of the edge protection structure 72 to the sheet 62 provides a further advantage in that it thickens the conductive sheet 62 around the perimeter. This localised mass increase improves the current capacity of the conductive sheet 62, particularly around the perimeter 70. This is particularly beneficial since it has been observed that lightning tends to strike surface protection layers around their perimeters.

Furthermore, the leading edge of the conductive sheet 62 may be prone to environmental erosion during use of the wind turbine blade. A further advantage of the edge protection structure 72 is that it will reduce environmental damage to the conductive sheet as it envelops the leading edge of the sheet.

In this embodiment the edge protection structure also includes tab portions 80a,80b that are folded outer sections of the first and second wall portions 78a,78b and which are directed inwardly towards the channel 76. The tab portions 80a,80b serve to eliminate any sharp parts along the wall portions by ensuring they are contained within the interior of the folded strip 74.

In applying the edge protection structure 72 to the conductive sheet 62, the strip 74 is pressed onto the edge 68 of the sheet and the first and second wall portions 68a,68b are pressed against the upper and lower surfaces 64,66 of the sheet 62, respectively. This is illustrated in Figure 7c. The inherent stiffness of the folded strip 74 may be sufficient to hold it in place securely enough as the component is handled into the blade mould layup. However, if further robustness is required the edge protection structure 72 may be fixed onto the sheet 62 by an additional technique. For example, a suitable thermoplastic hot melt adhesive could be used. Other glues are of course acceptable. It is also envisaged that the edge protection structure 72 could be welded to the sheet 62, for example spot welded at a plurality of spaced points 89, as shown in Figure 7b, or mechanical fasteners such as staples, eyelets or rivets may be used.

Conveniently the strip-like form of the edge protection structure 72 lends itself to being stored in coil form on a storage reel 90, as is shown in Figure 9. Therefore a required length of the edge protection structure 72 may be drawn from the reel 90 and cut to size during assembly of the preformed component.

Some variations on the specific embodiment described above with reference to Figures 5 to 7 have already been mentioned. However, the skilled person will appreciate that various other modifications may be made to the specific embodiments without departing from the scope of the invention as defined by the claims.

For example, Figures 8a to 8c illustrate alternative forms of edge protection structure 72. In more detail, the edge protection structure 72 in Figure 8a is a relatively rigid elongate strip that is rectangular in cross section and which defines a channel 76 along one of its short edges. So, this variant still has a relatively low profile in a similar way to the folded strip of the first embodiment. Such a form may be manufactured from a suitable metal and may be extruded through an appropriately shaped die.

Figures 8b and 8c show similar rigid elongate structures but the edge protection structure 72 of Figure 8b is generally circular in cross section whereas the edge protection structure 72 in Figure 8c is generally oval in cross section.