Abstract:
A method of making a wind turbine component of composite construction with enhanced radar absorbing properties is described. The method comprises making the component and then modifying the component by applying circuit analogue elements to a surface of the component.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to composite structures such as wind turbine blades that include radar-absorbing material. 
       BACKGROUND 
       [0002]    It is known to incorporate radar-absorbing material (RAM) into composite structures such as wind turbine blades. This is done to reduce the radar reflectivity of the blades so that they do not interfere with radar systems such as air traffic control systems or marine radar systems. 
         [0003]    Many radar-absorbing materials are based upon the Salisbury Screen, which comprises three layers: a lossless dielectric layer sandwiched between a reflector layer or ‘ground plane’ and an impedance layer or ‘lossy screen’. The lossless dielectric is of a precise thickness equal to a quarter of the wavelength of the radar wave to be absorbed; the ground plane comprises a layer of highly reflective conductive material such as metal or carbon; and the lossy screen is generally a thin resistive layer. 
         [0004]    Circuit analogue (CA) RAM technology has proven to be particularly effective for use in wind turbine blades. This is similar to the Salisbury Screen arrangement, but the impedance layer is replaced by a CA layer comprising an array of elements, such as monopoles, dipoles, loops, patches or other geometries. The elements form a pattern that repeats across the CA layer. The CA layer and the ground plane form a radar absorbing circuit in the composite structure. 
         [0005]    It is known to embed a RAM impedance layer within a laminated composite structure such as a wind turbine blade. For example,  FIG. 1   a  is a cross-section through an aerofoil part of a wind turbine blade  10 , between a leading edge  11  and a trailing edge  12 . The blade  10  is constructed from two aerodynamic shells, an upper shell  13  and a lower shell  14 , which are joined together at join lines or seams that extend along the leading and trailing edges  11 ,  12  respectively. The shells  13 ,  14  are formed from a glass fibre cloth and resin composite. The shells  13 ,  14  are supported by a tubular structural spar  15  formed from glass fibre and carbon fibre. 
         [0006]      FIG. 1   b  is an enlarged schematic view of the leading edge  11  of the blade  10 , in which various layers comprising the shells  13 ,  14  can be seen. For ease of illustration the layers are shown separated, but in reality adjacent layers would abut. The shells  13 ,  14  each comprise a skin  16  of composite construction and formed from one or more layers of glass-fibre fabric within a hardened resin matrix. A CA layer  17  is deposited on an outer surface of the skin  16 . A gel coat  18  covers the CA layer  17 . A ground plane  19  comprising a thin layer of carbon veil, is adhered to an inner surface of the skin  16  such that it is in spaced apart relation from the CA layer  17 . The CA layer  17  and the ground plane  19  act together to form a radar absorbing circuit. 
         [0007]    When constructing the blade  10 , each of the shells  13 ,  14  are moulded separately and then joined together. To make a shell  13  or  14 , the various glass-fibre fabric layers comprising the skin  16  are laid up in a gel-coated mould. The layers may be infused with resin in the mould, or the layers may be pre-impregnated with resin (prepreg). The resin is subsequently hardened in a curing process. The CA layer  17  is pre-printed or otherwise deposited on a surface of one of the glass-fibre fabric layers prior to layup so that the CA layer  17  becomes embedded within the resulting composite structure. A prepreg material suitable for use in the above-described moulding process to provide an embedded CA layer is described in WO2010/122350. The prepreg material comprises an impedance layer deposited onto a resin-impregnated glass-fibre layer. 
         [0008]    Whilst an embedded CA layer  17  works well in many cases, it has been found that this arrangement works less well at joins in a composite structure, for example at the join  20  between the upper and lower shells  13 ,  14  at the leading edge  11  of a wind turbine blade  10 . This is because the repeating pattern of the CA elements is inevitably disrupted at the join, which can result in reduced RAM performance. The present invention aims to overcome this problem. 
       SUMMARY OF THE INVENTION 
       [0009]    According to the present invention, there is provided a method of making a wind turbine component of composite construction with enhanced radar absorbing properties, wherein the method comprises making the component and then modifying the component by applying circuit analogue (CA) elements to a surface of the component. 
         [0010]    The component is preferably a rotor blade for a wind turbine, but it will be appreciated that the component may be any other wind turbine component, for example the nacelle or nose cone. 
         [0011]    If the component is made from multiple elements that are joined together, for example upper and lower blade shells, the elements may be joined prior to applying the CA elements. Hence, the step of making the component may include joining two elements together to form at least part of the component. The step of modifying the component may include applying the CA elements to a region of the surface that bridges an interface between the two elements. Providing the CA elements after the elements have been joined enables the CA elements to be applied as a repeating pattern that is not disrupted at the join between the elements. 
         [0012]    The CA elements may be provided on the surface of the component as one of the final stages in the manufacturing process, for example after the component has been moulded and cured. Hence, the step of making the component may include arranging in a mould one or more fibrous layers within a matrix material, and subsequently curing the matrix material. It will be appreciated that the component may be made using any other suitable composites manufacturing technique, for example extrusion or automated techniques including automated fibre placement (AFP) and automated tape laying (ATL). 
         [0013]    The fibrous layers may be any suitable fibrous layers used in the fabrication of composite articles, for example plies, mats or sheets of woven or non-woven fibres. The fibres themselves may be any suitable fibres, for example carbon or glass fibres. Typically glass fibres are used in the construction of rotor blades for modern wind turbines. The fibrous material may be a prepreg material, i.e. one in which the fibres are pre-impregnated with a semi-cured matrix material. Alternately, ‘dry’ fibrous material may be used, in which case resin infusion or other suitable techniques may be used to supply the matrix material to the mould. The matrix material is typically a polymeric resin, such as epoxy resin. 
         [0014]    The component may have a gel-coated outer surface. This may be achieved by providing a gel coat on a surface of the mould. The step of modifying the component may include applying the CA elements over the gel coated outer surface of the component. 
         [0015]    The process of making the component may include arranging a ground plane in spaced apart relation with the surface of the component to which the CA elements are to be applied. Arranging the ground plane may include adhering the ground plane to an inner surface of the component. Alternatively the ground plane may be embedded within the composite structure of the component. For example the ground plane may be laid up in the mould and integrally moulded with the component. The ground plane is made of conductive material such as metal or carbon and forms a radar absorbing circuit in combination with the CA elements. Preferably the ground plane is a layer of carbon veil. The carbon veil is preferably less than one millimetre thick. 
         [0016]    The method may include applying a protective layer over the CA elements to protect the CA elements from environmental conditions. For example, the method may involve painting over the CA elements. The gel coat may be partially ground off before the CA elements are applied. Grinding the gel coat provides a keying surface to ensure a strong bond between the paint and the gel coat. Alternatively, a film may be provided over the CA elements. Suitable films include paint-replacement films, which are commonly used in the automotive industry. Alternately, the CA elements may be sufficiently durable to withstand environmental conditions without needing to be covered. 
         [0017]    The CA elements are preferably applied to an outer surface of the structure. The CA elements may be applied directly to the outer surface. For example the method may comprise printing the CA elements on the surface. The CA elements may be printed using conductive ink, for example a carbon-based ink. 
         [0018]    As an alternative to printing the CA elements on the surface of the component, the CA elements may be carried by a film, which is applied to the surface. The CA elements may be pre-printed, or otherwise deposited, on a surface of the film. 
         [0019]    Accordingly, the invention also provides a film for applying to a surface of a component, wherein the film carries circuit analogue elements for enhancing the radar absorbing properties of the component. Preferably the CA elements are printed on a surface of the film. The film is preferably flexible. The film is preferably made from a plastics material, such as polyvinyl chloride (PVC), polyolefin, polycarbonate or any other suitable polymeric film. Preferably the film is a paint-replacement film of the type mentioned above. 
         [0020]    Conveniently, the film may have an adhesive surface to facilitate application of the film to the surface of the component. The adhesive may be a heat-activated adhesive or a pressure-sensitive adhesive. The CA elements may be carried on the adhesive surface of the film. This is advantageous because the film then covers the CA elements on the surface of the component and provides protection against environmental conditions. Alternatively, the CA elements may be provided on the opposite surface of the film. In this configuration the film would not protect the CA elements from environmental conditions, and so the CA elements would need to be intrinsically durable or they could be covered by a protective layer such as a paint layer or another film layer. 
         [0021]    The film may serve as an appliqué, and may be releasable from the CA elements. Hence the method of modifying the component may include removing the film from the surface of the component thus leaving the CA elements bonded to the surface. 
         [0022]    It will be appreciated that the CA elements may be applied to the surface of the component as a retrofit, for example some time after the component has been manufactured. Accordingly, the invention provides a method of enhancing the radar absorbing properties of a wind turbine component of composite construction, wherein the method comprises applying circuit analogue elements to a surface of the component after the component has been manufactured. Hence the method may comprise making the component and then modifying the component by applying the CA elements to the surface of the component. 
         [0023]    Expressed in apparatus terms, the present invention provides a wind turbine component of composite construction and having enhanced radar-absorbing properties, wherein circuit analogue elements are provided on an outer surface of the component. 
         [0024]    The component may have a gel-coated outer surface and the CA elements may be provided on top of the gel coat. The component may include a protective layer covering the CA elements. The component may comprise two elements that are joined together and the CA elements may be provided in a region of the outer surface that bridges an interface between the two elements. The CA elements may form a pattern that repeats without interruption in said region of the outer surface. The component may comprise a ground plane beneath the outer surface, such that the CA elements form a radar-absorbing circuit in combination with the ground plane. Preferably the structure is a rotor blade for a wind turbine. The CA elements may be provided at a leading edge of the rotor blade in a region bridging an interface between a first and a second shell of the blade. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    Reference has already been made to  FIGS. 1   a  and  1   b  of the accompanying drawings in which: 
           [0026]      FIG. 1   a  is a cross-section through an aerofoil part of a wind turbine blade, between a leading edge and a trailing edge; and 
           [0027]      FIG. 1   b  is an enlarged view of the leading edge portion of the blade of  FIG. 1   a.    
           [0028]    In order that the present invention may be more readily understood, reference will now be made, by way of example, to  FIGS. 2 to 7 , in which: 
           [0029]      FIG. 2  is a cross-section through an aerofoil part of a wind turbine blade in accordance with the present invention, between a leading edge and a trailing edge; 
           [0030]      FIG. 3  is an enlarged schematic view of the leading edge portion of the blade of  FIG. 2 , showing a circuit analogue layer provided on an outer surface of the blade; 
           [0031]      FIG. 4  shows a paint layer provided over the circuit analogue layer; 
           [0032]      FIG. 5   a  shows upper and lower shells of the blade prior to being joined together to form the blade of  FIG. 2 ; 
           [0033]      FIG. 5   b  shows a circuit analogue layer being applied to the leading edge of the blade after the shells have been joined together to form the blade; 
           [0034]      FIG. 6   a  shows an embodiment of the present invention in which the circuit analogue layer is carried on an inner surface of a film, which is applied to the blade; 
           [0035]      FIG. 6   b  shows an embodiment of the present invention in which the circuit analogue layer is carried on an outer surface of a film, which is applied to the blade; and 
           [0036]      FIGS. 7   a  to  7   d  show a further embodiment of the present invention in which the circuit analogue layer is carried on an appliqué film that is applied to the blade and then released from the circuit analogue elements. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]      FIG. 2  is a cross-section through an aerofoil part of a wind turbine blade  30  in accordance with the present invention. The blade  30  extends between a leading edge  32  and a trailing edge  34 , and is constructed from two aerodynamic shells, an upper shell  36  and a lower shell  38 . The shells  36 ,  38  are joined together at join lines or seams that extend along the leading and trailing edges  32 ,  34  respectively. The seam  40   a  at the leading edge  32  can be seen in  FIG. 3 . The shells  36 ,  38  are supported by a tubular structural spar  42  formed from glass fibre and carbon fibre. 
         [0038]      FIG. 3  is an enlarged schematic view of the leading edge  32  of the blade  30 , in which the various layers comprising the shells  36 ,  38  can be seen. For ease of illustration the layers are shown separated, but in reality adjacent layers would abut. Each shell has a GFRP skin  44  formed from one or more layers of glass-fibre fabric within a hardened epoxy resin matrix. A gel coat  46  covers the outer surface of the skin  44 . A ground plane  48  comprising a thin layer of carbon veil, is adhered to an inner surface of the skin  44 . 
         [0039]    A circuit analogue (CA) layer  50  is printed on the gel coat  46 . The CA layer  50  is spaced apart from the ground plane  48  and acts together with the ground plane  48  to form a radar-absorbing circuit. The CA layer  50  comprises a circuit in the form of a geometric pattern of CA elements, which are printed on the gel coat  46  using a conductive carbon-based ink. In this example, the circuit analogue elements are sufficiently durable to withstand environmental protection. However, for increased protection, a paint layer  52  may be provided over the CA elements of the CA layer  50 , as shown in  FIG. 4 . The paint layer  52  may cover the entire outer surface of the blade  30 . This is a standard finishing step in the manufacture of wind turbine blades. The gel coat  46  is partially ground off before the CA layer  50  is printed on the gel coat  46 . Grinding the gel coat  46  provides a keying surface for the paint layer  52 , which ensures a strong bond to the gel coat  46 . 
         [0040]    To make the wind turbine blade  30 , the upper and lower shells  36 ,  38  are moulded individually in respective gel-coated moulds. Each shell  36 ,  38  undergoes a curing process to harden the resin. Once the resin has hardened, the resulting gel-coated shells  36 ,  38  are self-supporting and can be removed from the moulds. Referring to  FIG. 5   a , the spar  42  is initially joined to the lower shell  38 . An adhesive is applied along the edges  54   a,    54   b  of the lower shell  38  that define the leading and trailing edges  32 ,  34  of the blade  30  respectively. The upper shell  36  is then lowered onto the lower shell  38 , and adhered to the lower shell  38  and to the spar  42  to form the blade  30 , as shown in  FIG. 5   b . As mentioned above, the resulting blade  30  has seams  40   a,    40   b  running in a spanwise direction along the leading and trailing edges  32 ,  34  where the upper and lower shells  36 ,  38  are joined. The position of the seams  40   a,    40   b  is represented by the dotted horizontal lines in the cross-sectional view of  FIG. 5   b.    
         [0041]    In an alternative manufacturing procedure known in the art, the upper and lower shells  36 ,  38  may be laid-up in separate mould parts, and then the two mould parts may be brought together to form a closed mould. A matrix material may then be supplied to the closed mould and the two halves  36 ,  38  of the blade  30  may be cured in a single process. This results in an integral blade  30  having a continuous outer surface that does not include a noticeable seam between the upper and lower shells  36 ,  38 . However, despite having a continuous outer surface, there would of course be an internal discontinuity or seam at the interface between the upper and lower shells  36 ,  38  where the various layers of the respective shells  36 ,  38  abut. 
         [0042]    Referring still to  FIG. 5   b , once the upper and lower shells  36 ,  38  have been joined together, the CA elements  50  are applied to the leading edge  32 . The CA elements are applied to a region of the leading edge  32  containing the seam  40   a  or discontinuity between the upper and lower shells  36 ,  38 , i.e. a region bridging the interface between the upper and lower shells  36 ,  38 . Applying the CA elements  50  to the leading edge  32  after the upper and lower shells  36 ,  38  have been joined together ensures that the repeating pattern of CA elements  50  is continuous, i.e. uninterrupted by the seam  40   a  or discontinuity between the upper and lower shells  36 ,  38 . 
         [0043]    In an alternative embodiment of the invention, rather than being printed directly on the outer surface of the blade  30 , the CA elements are pre-printed on a PVC paint-replacement film, which is then applied to the blade  30 . This embodiment will now be described with reference to  FIGS. 6   a  and  6   b.    
         [0044]    Referring to  FIG. 6   a , a PVC paint-replacement film  56  includes an adhesive inner surface  58  and a non-adhesive outer surface  60 . The adhesive inner surface  58  is pre-printed with a pattern of circuit analogue elements  62 . The film  56  is applied to the leading edge  32  of the blade  30  with the inner surface  58  of the film  56  adhering to the gel-coat  46  of the blade  30 . The adhesive film  56  facilitates application of the CA elements  62  to the leading edge  32 , and in this configuration the film  56  also serves as an external barrier that covers and protects the CA elements  62  from environmental conditions. The CA elements  62  form a pattern that repeats without interruption in a region of the leading edge  32  that bridges the seam  64   a  between the upper and lower shells  36 ,  38 . 
         [0045]      FIG. 6   b  shows a variant of this embodiment, in which the CA elements  62  are printed on the non-adhesive outer surface  60  of the film  56 . In this configuration, whilst facilitating application of the CA elements  62  to the leading edge  32 , the film  56  does not protect the CA elements  62  from environmental conditions. However, a layer of paint or another film layer could be provided over the CA elements  62  if required. The CA elements  62  may alternatively be sufficiently durable not to require protection. 
         [0046]      FIGS. 7   a - 7   d  show a further embodiment of the present invention, in which CA elements are carried on a film that is releasable from the CA elements, i.e. an appliqué film. Referring to  FIG. 7   a , an appliqué film  64  includes a plurality of CA elements  66  on an inner surface  68 . An adhesive layer  70  is applied on top of the gel coat  46  of the leading edge  32  of the blade  30 . The appliqué film  64  is then applied to the leading edge  32  of the blade  30 , with the inner surface  68  of the film  64  facing the leading edge  32 . In this way, the CA elements  66  adhere to the leading edge  32  as shown in  FIG. 7   b . Next, and referring to  FIG. 7   c , the appliqué film  64  is peeled away from the CA elements  66  to leave the CA elements  66  adhered to the leading edge  32  of the blade  30  as shown in  FIG. 7   d . The CA elements  66  form a pattern that repeats without interruption in a region of the leading edge  32  that bridges the seam  64   a  between the upper and lower shells  36 ,  38 . In a variant of this embodiment, the CA elements  66  may be adhesive, which would dispense with the need for an adhesive layer  70  being applied to the gel coat  46 . 
         [0047]    It will be appreciated that many modifications may be made to the specific examples described above without departing from the scope of the present invention as defined by the accompanying claims.