Abstract:
A method of making a wind turbine blade incorporating a lightning protection system, the method comprising: providing a wind turbine blade mould; arranging a protruding element in the mould; arranging an electrically conductive layer over the protruding element in the mould; arranging one or more structural layers and/or structural components over the electrically conductive layer; consolidating the layers under vacuum to form a blade shell having an integrated electrically conductive layer proximate an outer surface of the shell; separating the protruding element from the blade shell to define a recess in the outer surface of the shell, with the electrically conductive layer extending into the recess; providing an electrical component of the lightning protection system adjacent an inner surface of the shell; and electrically connecting the electrically conductive layer to the electrical component via a connecting member; wherein an end portion of the connecting member is housed in the recess such that a surface of the connecting member abuts the electrically conductive layer across an interface region inside the recess, and such that the end portion does not substantially protrude from the outer surface of the shell.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to wind turbine blades that incorporate a lightning protection system, and to a method of making the same. The invention also extends to a pre-formed component for use in making the wind turbine blades, and a method of manufacturing the pre-formed component. 
       BACKGROUND 
       [0002]    Lightning protection systems are typically integrated into wind turbine blades to address the problem of lightning strikes. An electrically conductive lightning receptor is arranged on or adjacent an outer surface of the blade to receive lightning that strikes the blade. Typically, the lightning receptor is electrically connected to a down cable that extends inside the blade. The lightning is discharged from the lightning receptor to the down cable and then to a ground potential via conductors that extend inside the blade, nacelle and tower of the wind turbine. Lightning protection systems therefore allow lightning to be discharged safely and minimise the risk of damage to the wind turbine from lightning strikes. 
         [0003]    For example, US 2011/0182731 describes a wind turbine blade in which receptors extend through the blade shell such that they are exposed at the outer and inner surfaces of the blade shell. At the inner surface of the blade shell, the receptor is connected to a down cable. To increase the effectiveness of the lightning protection system, a conducting layer is laid over the outer surface of the blade shell and hence over the lightning receptors. The conducting layer increases the area of the blade that can receive lightning, thereby increasing the rate at which the receptors capture lightning strikes. 
         [0004]    However, such wind turbine blades are cumbersome to manufacture because the conducting layer must be added to the blade after the blade shell has been manufactured, which requires an extra, time-consuming step in the manufacturing process. Furthermore, ridges or other such discontinuities of the outer blade surface in the region of the lightning receptors can adversely affect the aerodynamic design of the blade, leading to reduced aerodynamic efficiency. 
         [0005]    It is an object of the invention to mitigate or overcome these problems. 
       SUMMARY OF THE INVENTION 
       [0006]    Against this background, and from a first aspect, the invention resides in a method of making a wind turbine blade incorporating a lightning protection system, the method comprising: providing a wind turbine blade mould; arranging a protruding element in the mould; arranging an electrically conductive layer over the protruding element in the mould; arranging one or more structural layers and/or structural components over the electrically conductive layer; consolidating the layers under vacuum to form a blade shell having an integrated electrically conductive layer proximate an outer surface of the shell; separating the protruding element from the blade shell to define a recess in the outer surface of the shell, with the electrically conductive layer extending into the recess; providing an electrical component of the lightning protection system adjacent an inner surface of the shell; and electrically connecting the electrically conductive layer to the electrical component via a connecting member; wherein an end portion of the connecting member is housed in the recess such that a surface of the connecting member abuts the electrically conductive layer across an interface region inside the recess, and such that the end portion does not substantially protrude from the outer surface of the shell. 
         [0007]    In this way, the invention provides a method of forming a wind turbine blade incorporating a lightning protection system such that the connecting element is housed in the recess and does not protrude substantially from the outer surface of the blade. Thus, the connecting element does not adversely affect the aerodynamic performance of the blade, which ensures efficient generation of electricity. 
         [0008]    In a preferred embodiment, the protruding element and electrically conductive layer are provided together as a pre-formed component and are arranged in the mould simultaneously. This provides a fast and efficient method of arranging the components in the mould. 
         [0009]    So that the end portion of the protruding element may fit tightly into the recess, the protruding element may have substantially the same shape and dimensions as the end portion of the connecting element. 
         [0010]    The interface region may comprise a base of the recess. This maximises the surface area of the interface region, improving electrical contact between the connecting element and the electrically conductive layer. 
         [0011]    The method may further comprise forming an aperture through the blade shell in the base of the recess, and arranging the connecting member such that it extends through the aperture. In this way, a remaining portion of the connecting member can be housed in the aperture, to facilitate contact between the connecting member and the electrical component. 
         [0012]    In one embodiment, the protruding element includes a protruding detail formation, the detail formation providing a corresponding indentation in the base of the recess, and the method comprises forming the aperture at the indentation. The detail formation facilitates the step of forming the aperture, thereby improving efficiency of the method. 
         [0013]    To further improve the electrical contact at the interface region, the method may optionally comprise sealing between a portion of the electrically conductive layer and a surface of the protruding element to prevent contamination of the electrically conductive layer in the interface region. 
         [0014]    The sealing step may comprise disposing a protective layer between the electrically conductive layer and the surface of the protruding element. In this case, the protruding element, electrically conductive layer and protective layer may be provided together as a pre-formed component such that they are arranged in the mould simultaneously. 
         [0015]    To adhere the protective layer to the electrically conductive layer, the protective layer may be an adhesive layer. In this way, the protective layer may also serve to adhere the electrically conductive layer to the protruding element. In embodiments that include a protective layer, the method may further comprise removing the protective layer from the electrically conductive layer before arranging the connecting member in the recess. 
         [0016]    Alternatively or additionally to the protective layer, the protruding element may be at least partially formed from a resilient material, and the sealing step may comprise deforming the protruding element to form a seal between the electrically conductive layer and the surface of the protruding element. 
         [0017]    In preferred embodiments, the electrically conductive layer is a lightning receptor, which is optionally a foil or a mesh so as to further minimise disruption to the aerodynamic form of the blade. 
         [0018]    Preferably, the electrical component comprises a connector block for coupling the electrically conductive layer to an electrically conducting cable inside the blade. 
         [0019]    Optionally, the step of consolidating the layers under vacuum includes a resin infusion process. In an alternative embodiment, pre-preg layers may be used, in which case the resin infusion process may be omitted. Preferably, the step of consolidating the layers under vacuum includes a curing process. 
         [0020]    The invention also resides in a wind turbine blade incorporating a lightning protection system. The wind turbine blade comprises a blade shell having an inner surface and an outer surface; an integrated electrically conductive layer proximate the outer surface of the blade shell; an electrical component arranged adjacent the inner surface of the blade shell; and at least one connecting member electrically connecting the electrically conductive layer to the electrical component through the blade shell. The outer surface of the blade shell is provided with a moulded recess, the electrically conductive layer extending at least partially into said recess, and an end portion of the connecting member is housed in the recess such that a surface of the connecting member abuts the electrically conductive layer across an interface region inside the recess, and such that the end portion does not substantially protrude from the outer surface of the shell. 
         [0021]    Preferably, the interface region comprises a base of the recess. This maximises the surface area of the interface region, improving electrical contact between the connecting element and the electrically conductive layer. 
         [0022]    Preferably, a portion of the connecting member is arranged outboard of at least a portion of the electrically conductive layer relative to the inside of the wind turbine blade. 
         [0023]    In preferred embodiments, the electrically conductive layer is a lightning receptor, which is optionally a foil or a mesh so as to further minimise disruption to the aerodynamic form of the blade. 
         [0024]    To provide a high electrical conductivity while reducing cost, the electrically conductive layer is preferably made from aluminium. However, it will be appreciated that the electrically conductive layer may otherwise be made from copper or another suitable conductive material 
         [0025]    In preferred embodiments, the electrical component comprises a connector block for coupling the electrically conductive layer to an electrically conducting cable inside the blade. 
         [0026]    The invention also extends to a wind turbine comprising the wind turbine blade as described above, or a wind turbine blade made in accordance with any method described above, and to a wind farm comprising a plurality of such wind turbines. 
         [0027]    From another aspect, the invention resides in a pre-formed component for use in making a wind turbine according to the method described above, the pre-formed component comprising an electrically conductive layer and at least one protruding element, wherein a portion of the electrically conductive layer is shaped around the protruding element to define a recess in the electrically conductive layer, the recess comprising a base and a surface of the electrically conductive layer being sealed or sealable to the base. 
         [0028]    To provide multiple recesses such that the electrically conductive layer may receive multiple connecting elements, the component may comprise a plurality of protruding elements and the electrically conducting layer may comprise a plurality of recesses, each being shaped around a respective protruding element. 
         [0029]    To protect the base surface from contamination during the moulding process, a protective layer may be provided between the electrically conductive layer and the protruding element. 
         [0030]    In one embodiment, the protruding element is at least partially formed from a resilient material, and the electrically conducting layer is sealed or sealable to the base surface by deforming the protruding element during a vacuum-assisted moulding process. In this way, the protruding element protects the base surface from contamination during the moulding process. 
         [0031]    So that the end portion of the protruding element may fit tightly into the recess, the protruding element may have substantially the same shape and dimensions as an end portion of a connecting element, such as a head of a bolt. In a preferred embodiment, the protruding element is of substantially the shape and dimensions of a bolt head. 
         [0032]    Optionally, a surface of the protruding element may include a protruding detail formation for forming an indent in the blade surface. The indent may indicate a position for drilling an aperture to increase efficiency of the manufacturing process. 
         [0033]    The invention further extends to a method of forming the pre-formed component described above, the method comprising: providing a forming tool comprising a female element and a male element, the male element being of substantially the same shape and dimensions as a protruding element of the pre-formed component, arranging an electrically conductive layer of the pre-formed component between the male and female elements, pressing the male and female elements together so as to form a recess in the conductive layer, and removing the female element from the conductive layer. 
         [0034]    Preferably, the male forming element defines the protruding element of the pre-formed component. In this way, the male forming element may remain in the recess after fabrication of the pre-formed component. 
         [0035]    In an alternative embodiment, the method may comprise replacing the male forming element with a protruding element of the pre-formed component. In this way, a male forming element with properties suitable for moulding the recess may be selected for use in forming the pre-formed component, and a protruding element with properties suitable for protecting an interface region of the electrically conductive layer may be selected for use in forming the wind turbine blade. 
         [0036]    The method may further comprise sealing the protruding element to the conductive layer, so as to protect the conductive layer from contamination. 
         [0037]    Optionally, the method may further comprise arranging a protective layer between the male element and the conductive layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]    In order that the invention may be more readily understood, specific embodiments of the invention will now be described, by way of example only, with reference to the following drawings in which: 
           [0039]      FIG. 1A  is a perspective view of a wind turbine blade according to the invention; 
           [0040]      FIG. 1B  is a cross-sectional view of part of the wind turbine blade of  FIG. 1 , taken along the line A-A of  FIG. 1A ; 
           [0041]      FIG. 10  is a partial cross-sectional view of  FIG. 1B ; 
           [0042]      FIGS. 2A to 2H  illustrate a method of making a structural shell of the wind turbine blade of  FIGS. 1A to 10 ; 
           [0043]      FIGS. 3A and 3B  are plan views of pre-formed components for use in making a wind turbine blade according to the method illustrated in  FIG. 2A to 2H ; and 
           [0044]      FIGS. 4A to 4C  illustrate a method of making the pre-formed component of  FIG. 3A . 
       
    
    
     DETAILED DESCRIPTION 
       [0045]      FIG. 1A  illustrates a wind turbine blade  10  that incorporates a lightning protection system. The blade comprises a blade shell  12  formed from two half shells. The half shells are typically moulded from glass-fibre reinforced plastic (GRP), comprising glass fibres embedded in a cured resin matrix. The blade shell  12  has an outer surface  14  that is exposed to the blade surroundings, and an inner surface  16  (shown in  FIG. 1B ) that faces an internal cavity  18  of the blade  10 . 
         [0046]    Referring also to  FIG. 1B , the lightning protection system of the wind turbine blade  10  comprises an electrically conductive layer  20  that acts as a lightning receptor, an electrical component  22  in the form of a connector block, an electrically conducting cable  24  connected to the connector block  22  that acts as a down cable, and a connecting element  26  that electrically connects the electrically conductive layer  20  to the connector block  22 . 
         [0047]    The electrically conductive layer  20  is a metallic mesh or, as in the embodiment illustrated, a metallic foil made from a metal such as copper or aluminium. The foil  20  lies proximate, and is integrated with, the outer surface  14  of the blade shell  12 . Specifically, the foil  20  is integrated with the cured resin matrix comprising the blade shell  12 . In this way, the electrically conductive layer  20  does not disrupt the aerodynamic form of the blade shell  12 . 
         [0048]    The outer surface  14  of the blade shell  12  is provided with a plurality of recesses  28 . As best illustrated in  FIG. 10 , each recess  28  comprises a base surface  30  and a peripheral wall  32  that extends outwardly away from the base surface  30  towards the outer surface  14  of the shell  12 . The electrically conductive layer  20  extends across the outer surface  14  of the blade shell  12  and into the recess  28 . In this way, the electrically conductive layer  20  is integrated with the base surface  30  and peripheral wall  32  of the recess  28 . 
         [0049]    Each recess  28  houses an end portion  34  of a connecting element. In the embodiment described, the connecting element  26  is a metallic bolt and the end portion  34  is a bolt head such that the recess  28  houses the bolt head  34 . As best seen in  FIGS. 1A and 1B , each recess  28  is of substantially the same shape and dimensions as the bolt head  34 . The bolt head  34  is therefore fully accommodated in the recess  28 , and does not protrude substantially beyond an outer surface  36  of the blade  10 . Specifically, the bolt head  34  lies flush with the outer surface  36  of the blade  10  such that it does not disrupt the aerodynamic form of the blade  10 . 
         [0050]    At the base  30  of the recess  28 , the electrically conductive layer  20  is in electrical contact with the bolt head  34 . The base  30  of the recess  28  therefore defines an interface region between the bolt head  34  and the electrically conductive layer  20  at which electrical contact occurs. 
         [0051]    As shown in  FIG. 10 , in the base  30  of the recess  28 , an aperture  38  is provided in the blade shell  12  and the electrically conductive layer  20 . The aperture  38  extends through the thickness of the blade shell  12  such that the aperture  38  is open at both the base surface  30  of the recess  28  and the inner surface  16  of the blade shell  12 . The remaining portion of the connecting element  40  (i.e. the shaft of the bolt) is housed in the aperture  38 . In this way, the connecting element  26  extends through the aperture  38  from the recess  28  to the inner surface  16  of the blade shell  12 . 
         [0052]    Inside the blade  10 , and opposite the recess  28  in the outer surface  14  of the blade shell  12 , the connector block  22  is mounted to the inner surface  16  of the blade shell  12  by suitable means, such as an adhesive, and the conducting cable  24  is connected to the connector block  22 . The connector block  22  is arranged in contact with an inner end  42  of the connecting element  26  (i.e. with the end  42  of the shaft  40  of the bolt  26 ). In this way an electrically conducting path is formed that runs from the electrically conductive layer  20 , into the connecting element  26  via the interface region  30 , to the connector block  22  and into the electrically conducting cable  24 . 
         [0053]    When the wind turbine blade  10  is in use, lightning that strikes the blade  10  is received by the electrically conductive layer  20 . The current is conducted to the connecting element  26  through the conducting interface  30  between the electrically conductive layer  20  and the connecting element  26 , which is disposed in the recess  28 . The connecting element  26  conducts the current through the blade shell  12  to the connector block  22 . The current is then conducted to the electrically conducting cable  24 , and then to conductors in the nacelle and tower, which direct it to a ground potential. 
         [0054]    Because the bolt head  34  of the connecting element  26  is housed in the recess  28 , and does not protrude substantially from the outer surface  36  of the blade  10 , the connecting element  26  does not adversely affect the aerodynamic performance of the blade  10 . 
         [0055]    A method of making the wind turbine blade  10  described above will now be described with reference to  FIGS. 2A to 2H . As illustrated schematically in  FIG. 2A , a wind turbine blade mould  44  is firstly provided. The blade mould  44  is typically provided as two mould halves, in which two half shells are formed, each mould half comprising a moulding surface  46 . Once the half shells have been formed, the two mould halves are brought together and the half shells are joined to form the blade shell  12 . 
         [0056]    As illustrated in  FIG. 2B , a protruding element  48  and an electrically conductive layer  20 , provided together in the form of a pre-formed component  50  as illustrated in  FIG. 3A , are arranged in the blade mould  44  on the moulding surface  46 . Each pre-formed component  50  comprises an electrically conducting layer  20  and at least one protruding element  48 . The protruding element  48  is of the shape and dimensions of the desired recess  28  in the blade shell, and hence comprises a surface  52  that corresponds to the base surface  30  of the recess  28 , and a peripheral wall  54  that corresponds to the peripheral wall  32  of the recess  28 . Specifically, the protruding element  48  has substantially the same shape and dimensions as the bolt head  34  that will be accommodated in the recess  28  when the blade  10  is fully assembled. 
         [0057]    The majority of the electrically conductive layer  20  is of substantially planar configuration. However, a portion of the electrically conductive layer  20  is shaped around the protruding element  48 , specifically around its surface  52  and peripheral wall  54 , such that that portion of the electrically conductive layer  20  takes the form of the protruding element  48 . A protective layer  56  is disposed between the electrically conductive layer  20  and the surface  52  of the protruding element  48 , such that the protective layer  56  covers the portion of the electrically conductive layer  20  that forms the interface region  30 . The protective layer  56  has adhesive on both sides so that it adheres the electrically conductive layer  20  to the protruding element  48 . In this way, the protective layer  56  protects the interface region  30  from contamination during the moulding process. Specifically, it prevents resin infiltrating into the interface region  30  between the electrically conductive layer  20  and the protruding element  48 . 
         [0058]    Once the pre-formed components  50  have been arranged in the mould  44 , each half shell is ‘laid up’ by arranging various laminate layers  58  of the half shells in the respective mould halves over the pre-formed components  50 , as illustrated in  FIG. 2C . 
         [0059]    A layer of dry fibrous material is placed on the inner mould surface  46  over the pre-formed component  50 . Following a resin infusion process, this layer ultimately forms the outer skin of the blade  10 . The fibrous layer is arranged in continuous contact with the electrically conductive layer  20 , such that the outer skin is moulded to the shape of the electrically conductive layer  20 , and is hence shaped around the protruding element  48 . 
         [0060]    A layer of structural foam is introduced into the mould half on top of the fibrous layer, and a further layer of dry fibrous material is placed on the upper surface of the structural foam. Following the resin infusion process, this layer ultimately forms the inner skin of the blade  10 . Further components such as spar caps may also be incorporated into the blade shell  12 , between the fibrous layers. 
         [0061]    The components are covered with an airtight bag to form a sealed region that encapsulates all of the components. The sealed region is then evacuated using a vacuum pump. A supply of liquid resin is connected to the sealed region, and resin flows into the sealed region through a plurality of resin inlets, which are longitudinally spaced along the mould half. Resin infuses throughout the lay-up in a generally chordwise direction, between the components in the half shell. Resin also infuses between the outer fibrous layer and the electrically conductive layer  20 , such that the resin and the outer fibrous layer are moulded around the electrically conductive layer  20 . 
         [0062]    The pump continues to operate during a subsequent curing operation in which the mould assembly is heated so as to cure the resin, although during the curing process the vacuum pressure may be adjusted. The bags are then removed from the moulded half shells. 
         [0063]    Because the structural components  58  of the blade shell  12  are laid up around the pre-formed-component  50 , the electrically conductive layer  20  is effectively embedded into the outer skin as the resin is cured, and is therefore situated proximal to the outer surface  14  of the blade shell  12 . The outer surface  14  of the blade shell  12  adopts the shape of the electrically conductive layer  20 , which is shaped around the protruding element  48 . Thus, the protruding element  48  confers a recess  28  in the outer surface  14  of the blade shell  12 , which, during the moulding process, houses the protruding element  48 . 
         [0064]    During the resin infusion process, the protective layer  56  arranged between the electrically conductive layer  20  and the protruding element  48  protects the portion of the electrically conductive layer  20  that forms the interface region  30  from the infusing resin. The portion that is covered by the protective layer  56  is therefore substantially free from resin. 
         [0065]    After the curing process, and as illustrated in  FIG. 2D , the connector block  22  and conductive cable  24  are fitted to the inner surface  16  of the blade shell  12 , at a position opposite the protruding element  48 . The connector block  22  and conductive cable  24  are attached to the inner surface  16  by adhesive. 
         [0066]    The two half shells are then brought together by closing the mould  44 . One of the mould halves is lifted, upturned, and pivoted into position above the other mould half. The half shells are joined together by an adhesive, and the blade shell  12  is removed from the mould  44 , as illustrated in  FIG. 2E . If resin has infused between the mould  44  and the pre-formed component  50  during the resin infusion process, this resin, once cured, may hold the pre-formed component  50  in place as the blade shell  12  is removed from the mould  44 . Hence, when the blade shell  12  is removed from the mould  44 , the pre-formed component  50  may initially remain housed in the recess  28 . 
         [0067]    In the next stage, the protruding element  48  is removed from the recess  28 , as shown in  FIG. 2F , for example by means of a hand tool such as a chisel, or by abrasive means such as grinding or sanding. If cured resin is present around the protruding element  48  this may hinder removal of the protruding element  48 , and any such cured resin may therefore be removed, for example by grinding or sanding. The protective layer  56  is also removed from the base surface  30  for example by peeling or scraping, to expose the interface region that is substantially free from resin. 
         [0068]    Next, as shown in  FIG. 2G , the aperture  38  is drilled into the base  30  of the recess  28  at its central point. The aperture  38  is of a sufficient width to house the remaining portion  40  of the connecting member  26  (i.e. the shaft of the bolt). An inner wall  60  of the aperture  38  is tapped to provide a female screw thread that receives a male screw thread on the shaft  40  of the bolt  26 . 
         [0069]    Finally, as shown in  FIG. 2H , the bolt  26  is inserted into the recess  28  and through the aperture  38 , where it is held in place by the cooperative screw threads. The bolt head  34  is housed in the recess  28 , and is in electrical contact with the electrically conductive layer  20  in the recess  28  at the interface region  30 . The interface region  30  is substantially free from resin as has been described above, such that electrical contact between the bolt head  34  and the electrically conductive layer  20  is achieved cleanly across the interface region  30 . 
         [0070]    A method of forming the pre-formed component  50  above will now be described with reference to  FIGS. 4A to 4C . According to the method, the protruding element  48  acts as a male forming element that is co-operable with a female forming element  62  to shape the electrically conductive layer  20 . 
         [0071]    To form the pre-formed component  50 , an electrically conductive layer  20  is arranged between the protruding element  48  and the female forming element  62 , and an adhesive protective layer  56 , such as a plastics film coated with an adhesive layer, is also arranged between the electrically conductive layer  20  and the protruding element  48 , as shown in  FIG. 4A . The forming elements  48 ,  62  are moved together such that the protruding element  48  is received in the female forming element  62 , as shown in  FIG. 4B . The electrically conductive layer  20  is deformed and shaped around the protruding element  48 , and the adhesive protective layer  56  adheres the electrically conductive layer  20  to the protruding element  48 . The female forming element  62  is then removed, as shown in  FIG. 4C , to leave the pre-formed component  50 . 
         [0072]    In an alternative embodiment of the pre-formed component  50 , illustrated in  FIG. 3B , the protruding element  48  is provided with a protruding detail formation  64  on its surface  52 . The protruding detail formation  64  confers a corresponding detail on the base surface  30  of the recess  28  during the moulding process described above. In the embodiment shown the detail formation  64  is arranged at the centre of the protruding element  48 , and therefore confers a detail at the centre of the base surface  30  of the recess  28 . The location of the detail indicates the point at which the aperture  38  should be drilled. 
         [0073]    In a further alternative embodiment of the pre-formed component or of the method, the protection of the interface region of the electrically conductive layer is not provided by the protective layer, but is instead provided by the protruding element itself. The protruding element may be made from a resilient material, or may be coated with a layer of a resilient material such as a silicon rubber. When the protruding element is placed under vacuum pressure during the moulding process, the resilient material of the protruding element is deformed. Specifically, the protruding element is compressed. In this way, the protruding element seals against the electrically conductive layer, thereby preventing resin from infiltrating into this region. 
         [0074]    In this embodiment, a first protruding element made from a substantially non-deformable material may be used as the forming element to form the pre-formed component. Before the pre-formed component is inserted in the mould, the non-deformable protruding element may be replaced with a deformable protruding element made from a resilient material. The protective layer may be dispensed with, or it may still be included, for example to adhere the protruding element to the electrically conductive layer, thus further preventing contamination of the interface region. 
         [0075]    Many modifications may be made to the embodiments described above without departing from the scope of the invention as defined in the following claims. 
         [0076]    For example, in the embodiment described, the blade shell  12  is made using a dry fibre cloth, and resin is infused into the cloth during the moulding process. In an alternative embodiment, pre-preg materials may be used instead, and the resin infusion step may be omitted. 
         [0077]    The connecting element need not be a bolt but may take any suitable form, and the recess may be any shape that is suitable for housing an end portion of the connecting element. Although in the embodiments described above the connecting element  26  lies flush with the blade surface, it will be appreciated that the connecting element may protrude slightly from the blade surface, or may lie inwardly of the blade surface, but it is preferably arranged such that it does not adversely affect the aerodynamic properties of the blade. 
         [0078]    The electrically conductive layer may be made from any electrically conducting material, and need not be a mesh or foil, but may take any suitable form. The electrically conductive layer need not necessarily be the outermost layer of the blade shell. Typically, further layers such as a gel coat layer and/or a print layer may be applied on top of the electrically conductive layer.