Abstract:
A wind turbine component incorporating radar-absorbing material having increased compatibility with lightning protection systems is described. The radar absorbing material includes a ground plane having an electrical conductivity and/or a dielectric constant that is higher in the presence of an electric field having a frequency of 1 GHz and above than in the presence of an electric field having a frequency of 10 MHz and below. Suitable materials for the ground plane include ferroelectric and ferrimagnetic materials and percolating material combinations, all of which have frequency-dependent properties that can be tuned to make the ground plane highly reflective at radar frequencies and benign at lightning discharge frequencies.

Description:
FIELD OF THE INVENTION 
     The present invention relates to radar absorbing materials (RAM), and in particular to RAM suitable for incorporating into wind turbine components and having improved compatibility with lightning protection systems. 
     BACKGROUND 
     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. The frequency range of these radar signals is approximately 1-10 GHz, and hence the RAM incorporated in wind turbine blades is typically optimised to attenuate radar signals in this frequency range. 
     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. 
     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 a CA layer comprising an array of elements, such as monopoles, dipoles, loops, patches or other geometries. The CA layer and the ground plane form a radar-absorbing circuit in the composite structure. The RAM employed in modern wind turbine blades typically uses a thin layer of carbon tissue, also referred to as ‘carbon veil’, as the ground plane. 
     Experimental tests have shown that the conductive ground plane employed in RAM has the potential to interfere with lightning protection systems, such as those incorporated in wind turbine blades to protect the blades from damage caused by lightning strike events. To illustrate this problem, a typical lightning protection system of a wind turbine blade will now be described with reference to  FIGS. 1 a   - 1   d.    
       FIG. 1 a    is a plan view of a tip end  10  of a wind turbine blade  12 . A lightning receptor  13  comprising a metal disc  14  is located on a suction surface  16  of the blade  12 , near the tip  18  of the blade  12 . Referring to  FIG. 1 b   , which is a cross-sectional side view through the tip end  10  of the blade  12 , it can be seen that the metal disc  14  is the head of a bolt  20 . The bolt  20  is screwed into a conductive base  22 , which is implanted within the tip end  10  of the blade  12 . A similar bolt  24  is screwed into the opposite side of the base  22  to define a lightning receptor  26  on a pressure side  28  of the blade  12 , at the tip end  10 . 
     Referring still to  FIG. 1 b   , the base  22  is connected to a lightning cable  30  via a connector element  32 . The lightning cable  30  is earthed and extends longitudinally inside the blade  12 , in a span wise direction, to the blade root. The cable  30  is surrounded by an insulating sheath and is attached to the main spar  32  of the blade  12  to prevent potentially damaging flashover discharges to the spar  32  from occurring. 
     In addition to the lightning receptors  13 ,  26  at the tip end  10 , a series of secondary receptors  34  ( FIG. 1 c   ) are provided at intervals along the length of the blade  12 . Referring now to  FIG. 1 c   , which is a cross-section through an aerofoil part of the blade  12 , between a leading edge  36  and a trailing edge  38 , the secondary receptors  34  are also in the form of metal bolts  40  ( FIG. 1 d   ), which are screwed into respective receptor bases  42  ( FIG. 1 d   ) located adjacent an inner surface  44  of the blade shell  46 . The secondary receptors  34  are connected to the lightning cable  30  ( FIG. 1 c   ) via connecting straps  48  extending between the base  42  of the respective receptor  34  and the lightning cable  30 . 
     The lightning receptors  13 ,  26 ,  34  are designed to attract lightning strikes and channel electricity safely to ground via the lightning cable  30 . Lightning clouds induce an electric field around the lightning receptors  13 ,  26 ,  34 . The induced electric field is a low-frequency electric field, typically of the order of 10 MHz and below. 
     Referring to  FIG. 1 d   , which is an enlarged view of the circled part  50  of  FIG. 1 c   , a CA layer  52  is embedded within the composite structure of the blade shell  46 , at a location between the inner surface  44  of the shell  46  and an outer surface  54  of the shell  46 . A continuous carbon reflector layer  56 , which serves as the ground plane, is adhered to the inner surface  44  of the shell  46 . The lightning receptor  34  penetrates both the CA layer  52  and the carbon ground plane  56 . 
     As shown in  FIG. 1 d   , the base  42  of the lightning receptor  34  is close to, and in fact is in contact with, the conductive carbon ground plane  56 . In this arrangement, the conductive carbon ground plane  56  tends to distort and reduce the induced electric field around the lightning receptors  13 ,  26 ,  34  in the presence of a charged lightning cloud. This can degrade the performance of the lightning receptors  13 ,  26 ,  34 . Also, the conductive carbon ground plane  56  may be at a low potential, which presents a risk of potentially damaging flashover discharges occurring between the lightning cable  30  and the ground plane  56 , or even in extreme cases, lightning striking the ground plane  56  in preference to the lightning receptors  13 ,  26 ,  34 . 
     Against this background, it is an object of the present invention to provide RAM that is more compatible with lightning protection systems. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a wind turbine component incorporating radar absorbing material, wherein the radar absorbing material includes a ground plane having an electrical conductivity and/or a dielectric constant that is higher in the presence of an electric field having a frequency of 1 GHz and above than in the presence of an electric field having a frequency of 10 MHz and below. 
     The wind turbine component preferably includes a lightning protection system, or at least part of a lightning protection system, for example one or more lightning receptors. 
     The material is preferably highly conductive at frequencies of 1 GHz and above and acts as an electrical insulator at frequencies of 10 MHz and below. Highly conductive materials are good reflectors of electromagnetic waves. Selecting a material for the ground plane that is highly conductive in the presence of electric fields of 1 GHz and above ensures that the material will be a good reflector of most radar signals. Preferably the ground plane is optimised to reflect radar signals having a frequency in the range of 1-10 GHz, and more preferably in the range of 1-6 GHz, which includes most radar signals used for air traffic control and marine purposes. In addition, selecting a material that also exhibits low conductivity in the presence of electric fields having frequencies of 10 MHz and below ensures that the material is a poor conductor of electricity at lightning frequencies. This ensures that the ground plane does not interfere with the electric fields surrounding lightning receptors and hence does not detrimentally interfere with the performance of the lightning protection system. 
     Materials having a high dielectric constant are also good reflectors, but can interfere with the electric field surrounding lightning receptors and degrade the performance of the lightning protection system. Selecting a material for the ground plane having a relatively high dielectric constant at radar frequencies and a relatively low dielectric constant at lightning frequencies ensures optimal performance as a radar reflector whilst also ensuring compatibility with the lightning protection system. 
     The theory underpinning the invention will now be explained with reference to equations 1 to 3 below. 
     The reflection coefficient (R) of a radar signal at normal incidence upon an interface between materials 1 and 2 is given by equation 1 below, where Z 1  and Z 2  are the impedances of materials 1 and 2 respectively, calculated according to equation 2 below. 
                   R   =       (       Z   2     -     Z   1       )       (       Z   2     +     Z   1       )             1             Z   =       (     μ   ɛ     )       1   2             2             
where μ is the magnetic constant and ∈ is the dielectric constant of the material.
 
     If material 2 is a good conductor, Z 2  approaches zero in equation 1, and the reflection coefficient (R) can be approximated by equation 3 below. 
     
       
         
           
             
               
                 
                   R 
                   = 
                   
                     
                       
                         - 
                         
                           Z 
                           1 
                         
                       
                       
                         Z 
                         1 
                       
                     
                     = 
                     
                       - 
                       1 
                     
                   
                 
               
               
                 3 
               
             
           
         
       
     
     A reflection coefficient (R) of −1 equates to 100% reflection, with a 180-degree phase change. 
     If material 2 has a high dielectric constant (∈), which is significantly higher than its magnetic constant (μ), Z 2  will not be zero, but will be a small enough value to result in a suitably-high reflection coefficient (R). 
     Suitable materials for the ground plane include (i) ferroelectric materials; (ii) ferrimagnetic materials; and (iii) percolating material combinations. These materials may be tuned so that they are intrinsically only reflective at radar frequencies (typically 1-10 GHz for wind-turbine applications) but have benign properties at lightning discharge frequencies (10 MHz and below). 
     Preferably, a material is selected that has a suitably-high dielectric constant and/or a suitably-high conductivity in the presence of an electric field having a frequency of 1 GHz and above, i.e. radar frequencies. The material should be highly conductive at these frequencies, i.e. have a sheet resistance of approximately 0.02 Ω/sq (ohms per square) or less. The dielectric constant of the ground plane is preferably in the range of 80-120 in the presence of an electric field having a frequency of 1 GHz or above. More preferably, the dielectric constant is between 90-110, and preferably still, between 95-105 at frequencies of 1 GHz and above. 
     The ground plane material is selected to have a suitably-low conductivity and/or dielectric constant in the presence of electric fields of 10 MHz and below, i.e. lightning frequencies. The material preferably has a sheet resistance greater than approximately 100,000 Ω/sq at such frequencies. The dielectric constant is preferably as close as possible to the dielectric constant of other surrounding composites in the structure. Hence, the dielectric constant is preferably in the range of 1-10; more preferably, between 2-6, and preferably still between 3-5 at frequencies of 10 MHz and below. 
     Ferroelectric materials maintain a permanent electric polarization that can be reversed, or switched, in an external electric field. Examples of ferroelectric materials include barium titanate (BaTiO 3 ), lead titanate (PbTiO 3 ), sodium nitrite (NaNO 2 ) and lead zirconate titanate (PZT). It is known that the dielectric properties of some ferroelectric materials are frequency dependent. For example, the dielectric constant may increase with increasing frequency. The dielectric constant, or relative permittivity, may be relatively low at megahertz frequencies and relatively high at gigahertz frequencies. The reflectivity of the material increases as the dielectric constant increases. The material may be tuned to reflect radar frequencies (e.g. 1-10 GHz) more strongly than lightning frequencies (i.e. 10 MHz and below). 
     Many ferrimagnetic materials also have frequency-dependent dielectric constants and can be suitably selected to have a dielectric constant that increases with frequency in the same way as described above for ferroelectric materials. Examples of suitable ferrimagnetic materials include Iron(II,III) oxide, also known as magnetite (Fe 3 O 4 ); hexaferrites such as barium hexaferrite (BaFe 12 O 19 ), and other ferrites composed of iron oxides and elements such as aluminium, cobalt, nickel, manganese and zinc. 
     Advantageously, some ferroelectric and ferrimagnetic materials exhibit low electrical conductivity so that they do not interfere with lightning protection systems. 
     Suitable percolating material combinations for the ground plane include particles of conductive material dispersed in a non-metallic host. The higher the concentration of conductive material, the more reflective the material combination becomes. The host material may be a polymer matrix. The conductive material may include metal or carbon, for example carbon fibres, graphite or carbon nantotubes. The conductive material may be carbon black, which is relatively inexpensive. The properties of the percolating material combination may be tuned in accordance with percolation theory. For example, the material combination may have frequency-specific conductivity. The material combination is selected such that the conductivity is relatively low at megahertz frequencies to avoid interference with lightning receptors, and such that the conductivity is relatively high at gigahertz frequencies so that the ground plane is highly reflective to radar signals. The percolating combination may be tuned to exhibit resonant behaviour, such that the material is only conductive over a chosen frequency band. 
     Hence, a ground plane that exhibits low conductivity or has a low dielectric constant at such frequencies will have low electric field interactions with the lightning receptors at these frequencies, and will not reduce or otherwise interfere with the electric field around the lightning receptors. Therefore, the ground plane is more compatible with lightning protection systems. 
     In preferred embodiments of the invention, the wind turbine component is a rotor blade, and in particular it is a rotor blade incorporating one or more lightning receptors. It will of course be appreciated that the component may be any other part of a wind turbine liable to reflect radar signals. For example, the component may be a rotor hub, a nacelle or a tower. The inventive concept includes a wind turbine having said component and a wind farm including said wind turbine. 
     The invention also provides, within the same inventive concept, a radar-reflecting ground plane for incorporating into a composite structure, the ground plane having an electrical conductivity and/or a dielectric constant that is higher in the presence of an electric field having a frequency of 1 GHz and above than in the presence of an electric field having a frequency of 10 MHz and below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference has already been made to  FIGS. 1 a -1 d    of the accompanying drawings in which: 
         FIG. 1 a    is a plan view of a tip end of a wind turbine blade; 
         FIG. 1 b    is a cross-sectional side view through the tip end of the blade; 
         FIG. 1 c    is a cross-section through an aerofoil part of the blade, between a leading edge and a trailing edge; and 
         FIG. 1 d    is an enlarged view of part of  FIG. 1   c.    
       In order that the present invention may be more readily understood, embodiments of the invention will now be described, by way of example only, with reference to  FIGS. 2 and 3 , in which: 
         FIG. 2 a    is a cross-section through an aerofoil part of a wind turbine blade in accordance with the present invention; 
         FIG. 2 b    is an enlarged view of part of  FIG. 2 a   , showing a RAM ground plane in accordance with the present invention; 
         FIG. 3 a    is a plot of the relative permittivity versus frequency, of a ground plane made of ferroelectric material tuned to become reflective at radar frequencies; 
         FIG. 3 b    is a plot of the relative permittivity versus frequency, of a ground plane made of ferrimagnetic material tuned to become reflective at radar frequencies; and 
         FIG. 3 c    is a plot of the conductivity versus frequency, of a ground plane made of a percolating material combination tuned to become highly conductive at radar frequencies. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2 a    shows a cross section of a wind turbine blade  60  in accordance with the present invention. The blade  60  is constructed from two aerodynamic shells, an upper shell  62  and a lower shell  64 , which are formed from a glass fibre cloth and resin composite. The shells  62 ,  64  are supported by a tubular structural spar  66  formed from glass fibre and carbon fibre. 
     The spar  66  forms the primary strengthening structure of the blade  60 . At the rear of each shell  62 ,  64  and towards the trailing edge  68  of the blade  60 , the shells  62 ,  64  are formed with a sandwich-panel construction, in which a foam core  70  is bonded between an inner sheet  72  and an outer sheet  74  of glass fibre. These sheets  72 ,  74  are also known as ‘skins’. The foam core  70  is used to separate the glass fibre skins  72 ,  74  and to keep the shell  62 ,  64  stiff in this region. 
     The wind turbine blade  60  incorporates a lightning protection system in the form of a first series of lightning receptors  76  on a suction surface  78  of the blade  60 , and a second series of lightning receptors  80  on a pressure surface  82  of the blade  60 . The lightning receptors  76 ,  80  in each series are located at five-meter intervals along the length of the blade  60 . Two lightning receptors  76 ,  80  are visible in the cross-sectional view of  FIG. 2 a   . The lightning receptors  76 ,  80  are attached, via conducting straps  84 , to a cable  86 , which runs longitudinally inside the blade  60  and is earthed. 
       FIG. 2 b    is a close-up view of one of the lightning receptors  80  shown in  FIG. 2 a   , on the pressure surface  82  of the blade  60 . The lightning receptor  80  is located within an aperture  88  in the outer skin  74 , and screws into a base  90  located inside the blade  60 . The composite structure of the wind turbine blade shell  64  includes radar absorbing material (RAM) in the form of a circuit analogue (CA) layer  92  and a ground plane  94  in spaced-apart relation. As shown in  FIG. 2 b   , the base  90  of the lightning receptor  80  abuts an inner surface  96  of the ground plane  94 . 
     Referring still to  FIG. 2 b   , The CA layer  92  comprises a circuit printed in the form of a pattern using carbon ink on a layer of E-glass. The CA layer  92  is embedded within the shell structure  64 , and is provided inwardly of the outer skin  74 . In this example, the ground plane  94  is provided inwardly of the inner skin  72 , and comprises a layer of barium titanate (BaTiO 3 ), which is a ferroelectric material. The BaTiO 3  layer  94  is painted onto an inner surface of the inner skin  72 . Various other techniques of applying the BaTiO 3  layer are outlined at the end of this description. 
     Referring to  FIG. 3 a   , this is a plot of the relative permittivity (dielectric constant) of the ferroelectric ground plane  94  versus frequency. The dielectric constant of the ground plane  94  is frequency dependent. At frequencies of approximately 10 MHz and below, which correspond to the frequencies of the induced electric fields surrounding the lightning receptors in the presence of a charged lightning cloud, the dielectric constant of the ground plane  94  is relatively low, circa  3 . When the dielectric constant is low, the ferroelectric material comprising the ground plane  94  does not affect the electric fields surrounding the lightning receptors  76 ,  80 , hence the material is compatible with lightning protection systems. Conversely, at frequencies above approximately 1 GHz, which are typical of radar signals, the dielectric constant is relatively high, of the order of 100. The higher the dielectric constant, the more reflective the ground plane  94  becomes, by virtue of equations 1 and 2 above. Hence, the ground plane  94  in this example is optimised to reflect most radar frequencies. 
     In another embodiment of the invention, the ground plane  94  comprises a film of Iron(II,III) oxide, also known as magnetite (Fe 3 O 4 ), which is a crystalline ferrimagnetic material. Referring to  FIG. 3 b   , this is a plot of the dielectric constant of the ferrimagnetic ground plane  94  versus frequency. As with the previous example, the dielectric constant of the ground plane  94  is frequency dependent. At frequencies of approximately 10 MHz and below, which correspond to the frequency of the electric field surrounding the lightning receptors  76 ,  80  in the presence of a charged lightning cloud, the dielectric constant is relatively low, circa  3 . Conversely, at frequencies of approximately 1 GHz and above, which are typical of most radar signals of interest for wind turbine applications, the dielectric constant is relatively high, of the order of 100. For reasons already described above, the ground plane  94  is optimised to reflect most radar signals, whilst also being compatible with lightning protection systems. 
     In a further embodiment of the invention, the ground plane  94  comprises a percolating material combination, in which particles of carbon black are dispersed within an epoxy resin matrix host. Referring to  FIG. 3 c   , this is a plot of the conductivity of the ground plane  94  versus frequency. The conductivity of the ground plane  94  is frequency dependent. At frequencies of approximately 10 MHz and below, which correspond to the frequencies of the electric fields surrounding the lightning receptors in the presence of a charged lightning cloud, the conductivity is relatively low. 
     The properties of the percolating combination are tuned so that the conductivity exhibits a resonance peak at a particular frequency or over a particular frequency band. In this example, the maximum conductivity occurs over a frequency band of 1 to 10 GHz. The higher the conductivity, the more reflective the ground plane becomes, by virtue of equations 1 and 2 above. Hence, the ground plane  94  is optimised to reflect radar frequencies of 1 to 10 GHz. As conductivity is low at frequencies of 10 MHz and below, the ground plane  94  does not interfere with the electric fields surrounding the lightning receptors  76 ,  80 , and hence is compatible with the lightning protection system. 
     It will be appreciated that many modifications may be made to the specific examples described above without departing from the scope of the invention as defined by the accompanying claims. In particular the ground plane  94  may be made from any material that is suitably reflective at radar frequencies and has suitably-low conductivity at much lower frequencies to ensure compatibility with lightning protection systems in accordance with the theoretical considerations presented herein. 
     Also, the location of the ground plane  94  within the composite structure  64  may differ from that shown in the accompanying drawings. The main consideration here is to ensure suitable separation between the ground plane  94  and the CA layer  92  so that the RAM is optimised to attenuate radar signals of a desired frequency. Whilst in the examples described above, the ground plane  94  is applied to the inner surface of the inner skin  72 , the ground plane  94  may instead be provided outwardly of the inner skin  72 , for example to the outer surface of the inner skin  72 . 
     The frequency-tuned materials comprising the ground plane  94  may conveniently be employed as particles dispersed in a carrier matrix, for example to form a paint or film layer. Hence, the ground plane  94  may be painted onto the inner or outer surface of the inner skin  72  or applied to the relevant surface in the form of a suitably-loaded polymer film. Alternatively, the ground plane  94  may be integrally formed with a composite skin, such as the inner skin  72 . For example, the inner skin  72  may be moulded from a resin loaded with a suitable ferroelectric or ferrimagnetic material or loaded with conductive particles to form a percolating material combination. In yet further embodiments, the frequency-tuned materials may be incorporated into a fabric or otherwise applied to a fabric, which may be laid up as part of the composite skin. It will of course be appreciated that, in other embodiments, the CA layer  92  may be replaced with an alternative impedance layer.