GB2503260A

GB2503260A - Wind turbine component with radar transmitting and absorbing parts

Info

Publication number: GB2503260A
Application number: GB1210960.9A
Authority: GB
Inventors: Stephen Appleton
Original assignee: Vestas Wind Systems AS
Current assignee: Vestas Wind Systems AS
Priority date: 2012-06-21
Filing date: 2012-06-21
Publication date: 2013-12-25
Also published as: GB201210960D0; WO2013190288A1

Abstract

Wind turbine component 50 has an outer shell 11 defining an internal cavity 37, where at least a part of the outer shell is adapted to transmit incident radar signals 52a into the cavity and where one or more reflective components 15 inside the cavity are adapted to absorb radar signals. Preferably the outer shell has a frequency selective surface embedded within it. Radar absorbing material, such as a radar absorbing panel 54, may be applied to the reflective components and the reflective component may be a metallic part of a lightning protection system. Preferably the reflective component is a cable 14 attached to a surface and the panel is omega shaped in cross section such that the round part of the omega profile covering the cable and legs of the omega being bonded to the surface. A method of reducing the radar cross section is also claimed.

Description

lmvovements in or relating to comr)osite structures

Field of the invention

The present invention relates to composite structures such as wind turbine blades that include radar absorbing materials (RAM) for reducing the radar cross section (RCS) of the composite structure.

Background

A known problem with wind turbines is that they may interfere with radar systems and cause unwanted events (known as clutter') to appear on the screen of the radar operator. This is because some wind turbine components, for example the tower, the hub and the blades are highly reflective to radar signals (i.e. electromagnetic radiation of microwave frequency). For example, as the blades move, they tend to produce a radar signature similar to that of aircraft, which can make it difficult for air traffic control and other radar operators to distinguish between aircraft and wind turbines.

Existing wind turbine blades are generally manufactured from reinforced composite materials. By way of background, Figure 1 shows a cross section of a prior art wind turbine blade 10. The blade 10 is constructed from two aerodynamic shells, an upper shell 11 and a lower shell 12 which are formed from a glass fibre cloth and resin composite. The shells 11 and 12 are supported by a tubular structural spar 13 formed from glass fibre and carbon fibre. A metal cable 14 of a lightning protection system is attached to a shear web 15 of the spar 13, and runs longitudinally inside the blade 10.

Whilst not shown in Figure 1, the cable 14 is attached via straps to a plurality of lightning receptors arranged at the surface of the blade 10.

The spar 13 forms the primary strengthening structure of the blade 10. At the rear of each shell 11 and 12 towards the trailing edge 16 of the blade 10, the shells 11, 12 are formed with a sandwich panel construction, in which a foam core 17 is positioned between an outer skin 18 and an inner skin 19 of glass fibre reinforced plastic (GRP).

The foam core 17 is used to separate the GRP skins 18 and 19 to keep the shell stiff in this region. The shells 11, 12 also include sandwich panels 20 located near the leading edge 21 of the blade 10.

As shown in Figure 1, an incoming radar signal 22 is incident upon the upper shell 11 of the blade 10. The incoming radar signal 22 (referred to hereafter as the incident signal') is partially reflected 23 by the upper shell 11. The extent to which the incident signal 22 is reflected depends upon the thickness and construction of the skin 18 and on the frequency, incidence angle and polarisation of the incident signal 22. The remainder of the incident signal 22 enters the shell 11 and a small proportion of its energy is absorbed due to the intrinsic loss properties of the GRP and the foam core. The majority of the incident signal 22 penetrates the shell 11 of the blade 10 and is reflected 24 by the shear web 15, the lightning cable 14 and other metallic components of the lightning protection system, and by the inner skins 19 of the shells 11, 12. This is an example of radar-reflecting components, but it will be appreciated that other radar-reflecting objects or materials may be present inside the blade.

In order to minimise the impact of wind turbines on radar systems, it is known to apply radar absorbing materials (RAM) to parts of the wind turbine to reduce the radar cross section (RCS) of those parts. For example W02010/122351 and W020101122352 each describe incorporating RAM within the sandwich panel regions of the blades.

A prior art wind turbine blade 30 with RAM incorporated in the sandwich panel regions 31 is shown in Figure 2. In this case, the reflected radar signal 32 is significantly attenuated by the RAM, typically by twenty decibels or more. In addition, the RAM prevents the incident radar signal 33 from penetrating the shells 11, 12 of the blade 30, and hence reflections from the shear webs 15, the lightning cable, the inner skins 19 of the shells 11, 12, and any other radar-reflecting objects or materials inside the blade are avoided.

Further details of the RAM will now be explained briefly with reference to Figure 3.

Figure 3 shows an exploded sectional perspective view of part of a sandwich panel region 31 of the prior art blade 30 of Figure 2. The sandwich panel 31 comprises the foam core 17, which has an inner surface 34 and an outer surface 35. The core 17 is disposed between the inner skin 19 and the outer skin 18. The outer surface 35 of the core 17 and the outer skin 18 face towards an exterior surface 36 (Figure 2) of the blade 30, whilst the inner surface 34 of the core 17 and the inner skin 19 face towards an interior cavity 37 (Figure 2) of the blade 30.

Referring still to Figure 3, an impedance layer 38 is provided on the outer skin 18, and a conductive ground plane 39, which functions as a radar reflecting layer, is provided between the core 17 and the inner skin 19. The foam core 17 serves as a dielectric layer between the ground plane 39 and the impedance layer 38.

In this example, the impedance layer 38 is a circuit analogue' (CA) layer, which comprises a carbon-ink circuit printed on a layer of glass-fibre which is embedded within the outer skin 18. The carbon-ink circuit is represented by the array of dashes in Figure 3. For the avoidance of doubt, the outer skin 18 has been made transparent in Figure 3 so that the CA layer 38 can be seen; in reality, the CA layer 38 would not be visible through the outer skin 18. The CA layer 38 forms a radar absorbing circuit in combination with the ground plane 39. When radar waves are incident upon the blade 30, the combination of the CA layer 38 and the ground plane 39 act to absorb the radar waves so that they are not reflected back to the radar source. In other examples, an otherwise resistive layer may be used in place of the CA layer 38.

Different regions of a wind turbine blade are subject to different forces. Consequently, sandwich panels at different locations within the blade structure may require different core thicknesses. Typically, the core thickness ranges from 5 mm to 50 mm.

The separation between the impedance layer 38 and the ground plane 39 is a key parameter for radar absorption performance, and must be carefully controlled to achieve a blade 30 having the desired absorption properties. Such careful control of the separation of these layers is made more difficult by the varying geometry of the blade 30, specifically the abovementioned variation in core thickness. In some parts of the blade 30, the sandwich panel 31 is simply too thin in comparison to the wavelength of the radar signal 33 to achieve effective RAM performance. In other pads of the blade 30, if the thickness of the sandwich panel approaches half the wavelength of the radar signal 33, a condition ensues at which attenuation of the radar signal 33 is not achievable. For radar frequencies of approximately three gigahertz, this thickness has been determined to be approximately 50 mm, but will depend upon the dielectric and magnetic constants of the materials comprising the core 17 and the inner and outer skins 19, 18.

A solution to this problem is described in W02010/122351 and W02010/122352, and involves the use of sandwich panels having a split-core. The split core divides the thickness of the core between inner and outer core layers disposed about an intermediate ground plane. The thickness of the outer core layer determines the separation between the impedance layer and the ground plane, whilst the overall core thickness can be varied by suitable selection of the thickness of the inner core layer. As the inner core layer is behind the ground plane, it does not affect the performance of the RAM. Accordingly, this split core arrangement allows the overall thickness of the core to be varied without affecting the performance of the RAM and hence the RAM is able to provide consistent radar absorption performance in structures where core thickness varies.

Whilst the split core RAM performs well, various disadvantages exist. For example, the split cores are more expensive to manufacture than standard cores and the split cores are heavier than standard cores, which is generally undesirable because blades should be as light as possible. Split cores are also unsuitable for certain regions of a blade, for example where the thickness of the composite is much less than the wavelength of the radar signals. For such regions the thickness may make it difficult to achieve high performance RAM.

Against this background, the present invention aims to provide an alternative solution for incorporating RAM in wind turbine components, and in particular in wind turbine blades.

Summarq of the ixesent invention The present invention provides a wind turbine component comprising an outer shell defining an internal cavity, wherein at least a part of the outer shell is adapted to transmit incident radar signals into the cavity, and one or more reflective components inside the cavity are adapted to absorb radar signals.

In preferred embodiments of the present invention, the wind turbine component is a wind turbine blade. However, it will be appreciated that the inventive concept can readily be applied to other components of a wind turbine, for example the nacelle, the hub or parts of the wind turbine tower.

Rather than attenuating or absorbing the incident radar signals, the shell is adapted in part or in whole to transmit the radar signals, i.e. to allow the radar signals to pass through the shell and into the cavity. The otherwise reflective components inside the cavity are adapted to absorb the transmitted radar signals. For example, RAM may be applied to these components and/or RAM may be incorporated within their structure or composition.

The present invention therefore provides an alternative solution to split core RAM panels when the composite thickness approaches half the wavelength of the incident radar signals. Consequently the present invention oveicomes the aforementioned disadvantages of split core panels. The present invention may also conveniently be employed in cases where the composite thickness is much smaller than the wavelength of the incident radar signals.

Whilst the outel shell of a prior art blade would inevitably transmit a proportion of incident radar signals, the adapted part of the outer shell of the present invention is adapted specifically to increase the transmission of radar signals, and preferably to maximise the transmission of iadar signals.

In preferred embodiments of the present invention, the adapted part of the outer shell is of sandwich panel construction and comprises a layer of core material, which is disposed between innel and outer skin layers. Typically the coie is a lightweight mateiial such as foam or balsa. The skin layers are typically made from GRP or other suitable composite material. However! it will be appreciated that the inventive concept may be applied more generally to non-sandwich panel regions.

The adapted part of the outer shell may incorporate a frequency selective surface (ESS).

Frequency selective surfaces, which are also referred to as dichroic surfaces', are known in the art and generally complise a patterned array of conductive material, which is applied to a suitable substrate. A FSS can be suitably tuned' to reflect or transmit electromagnetic radiation of a particular frequency or in a particular frequency range. In this case, the FSS is suitably tuned to transmit radar signals, and preferably tuned to maximise the transmission of radai signals. The radai signals comprise microwave radiation, and for the purposes of wind turbines, the ESS is preferably tuned to maximise transmission of radar frequencies between 1 and 10 GHz. Tuning the FSS iefers to selecting a suitable pattern for the conductive array in accordance with known techniques.

The conductive material of the ESS may be applied to the substrate in the form of a metallic ink, such as a silver-or nickel-based ink, or in the form of a high conductivity carbon-based ink. The ink may be printed onto a suiface of the substrate. The substrate may be a layer of glass fabric, which can be incoipoiated into the composite layup of the shell. Alternatively, the substrate may be a surface of a sandwich panel core, or the FSS may be printed directly onto an outer or inner surface of the shell after the shell has been cured.

Rather than printing the ESS, it may alternatively be formed on the substrate by other means. For example the FSS may be formed by etching the patterned array on a sheet of metallised polymer. Such metallised sheets are readily available and generally comprise a thin sheet of polymer material, typically polyester, which includes a metal coating, typically of copper or aluminium. The sheet may be bonded to a surface of the shell, and preferably the inner surface to avoid disrupting the aerodynamic properties or the appearance of the blade, and to protect the FSS from the harsh outside environment.

Preferably the sheet is bonded to the shell after the shell has been cured.

The reflective components inside the cavity are components that generally reflect incident radar signals were it not for the adaptation in accordance with the present invention. Typically components made from metal or carbon-fibre reinforced plastic (CFRP) strongly reflect radar signals GRP can also be highly reflective depending upon its thickness and the wavelength of the radar signals. In the case of a wind turbine blade, the reflective components may include parts of the spar such as a shear web; the lightning cable or other metal components of the lightning protection system such as the straps; and the inner skin of the blade, which is typically made from GRP.

Composite components inside the cavity, such as the shear webs for example, can be adapted to absorb or attenuate radar signals by incorporating RAM into their composite structure. One way of doing this is to add radar absorbing filler particles to the matrix material when the components are manufactured. The radar absorbing filler particles may include carbon nanotubes and other carbon particulates, ferrites and lossy dielectrics.

Alternatively or additionally, RAM may be applied to the otherwise reflective components inside the cavity after these components have been manufactured. For example, RAM panels may be applied to the components. The RAM panels preferably comprise a lightweight dielectric material such as foam. The foam may include radar absorbing particles such as the filler particles described above.

Alternatively, the panels may be adapted in a similar way to the radar absorbing sandwich panels described above by way of background to the present invention. So, for example, an impedance layer, such as a circuit analogue layer, may be applied to a first surface of the panel. A conductive ground plane, such as a layer of carbon veil, may be applied to a second surface of the panel, opposite the first surface. In this way, the panel forms a circuit analogue absorber, and will absorb radar signals that are incident upon the panel.

The RAM panels may be applied to composite components such as a shear web, or metal components such as the lightning cable, or other parts of the lightning protection system such as the metal straps that connect the lightning receptors to the lightning cable. When the RAM panel is applied to metal or otherwise conductive components, the component itself may act as the conductive ground plane of the absorber. In a particular embodiment of the present invention, a RAM panel comprising a foam body with a CA pattern printed on the surface of the foam is applied to the lightning cable, and the lightning cable acts as the ground plane. This avoids the need to provide a separate ground plane and hence avoids the risk of arcing which may occur when conductive components are placed near components of the lightning protection system. In this example, the shear web may incorporate RAM in the form of radar absorbing filler particles, and so it will be appreciated that only a relatively small panel is required, which is sufficient to cover the lightning cable. A panel having an omega-shaped cross section is preferred, as this is able to suitably surround the cable whilst the legs of the omega can be bonded to the shear web.

The lightning receptors in prior art blades are held in place by foam mounts. A suitable impedance layer such as a CA layer may be applied to these foam mounts and the metallic straps may serve as the ground plane. This conveniently allows the RCS of existing components to be reduced without requiring any substantial modification to the structure or arrangement ot these components.

Within the same inventive concept there is also provided a wind turbine comprising the wind turbine component described above, and a wind farm comprising such a wind turbine.

The invention also provides a method of reducing the radar cross section of a wind turbine component, the component comprising an outer shell defining an internal cavity, and the method comprising optimising the transmission of incident radar signals through at least part of the outer shell and into the cavity and optimising the absorption of the radar signals by components inside the cavity.

Optional and preferred features described above in relation to the apparatus apply equally to the method.

The method may comprise adapting the outer shell to optimise the transmission of incident radar signals through the shell and into the cavity. The method may comprise applying a suitably-tuned FSS to the outer shell. The method may comprise applying the FSS to a layer of glass fabric and incorporating said layer of glass fabric in the laminate layup of the outer shell during manufacture of the shell. The method may comprise applying the FSS to an inner surface of the outer shell after the shell has been manufactured. The method may comprise incorporating radar absorbing particles within the structure of the reflective component during manufacture of said component. The method may comprise applying RAM to the reflective component inside the cavity. For example, the method may comprise applying a RAM panel to one or more surfaces of the component.

Within the same inventive concept there is provided a wind turbine blade optimised according to the above method.

Brief description of the drawings

Reference has already been made to Figures ito 3 by way of background, in which: Figure 1 shows a cross section of a prior art wind turbine blade; Figure 2 shows a cross section of a prior art wind turbine blade incorporating radar absorbing material within a sandwich panel region of the shell of the blade; and Figure 3 shows an exploded sectional perspective view of part of the sandwich

panel region of the prior art blade of Figure 2.

In order that the present invention may be more readily understood, reference will now be made to the following drawings, in which: Figure 4 shows a cross section of a wind turbine blade according to a first embodiment of the present invention; Figure 4A is an enlarged schematic view of part of a sandwich panel of the blade of Figure 4; and Figure 5 shows a cross section of a wind turbine blade according to a second embodiment of the present invention.

Detailed description

In the following description, the same reference numerals are used to identify features that correspond to features which have already been described above in relation to Figures 1 and 2.

Referring to Figure 4, a wind turbine blade 50 according to a first embodiment of the present invention has a basic construction similar to the wind turbine blade 10 described by way of background with reference to Figure 1. However, the wind turbine blade 50 has been modified so that part of the outer shell 11, 12 of the blade 50 transmits radar signals 52a, 52b, i.e. rather than absorbing or reflecting the radar signals 52a, 52b, the modified shell 11, 12 allows the radar signals 52a, 52b to pass through into the internal cavity 37 of the blade 50.

Otherwise reflective components inside the cavity 37 are modified or adapted to absorb the incident radar signals 52a, 52b, i.e. adapted to have a reduced radar cross section (RCS). In this embodiment, a radar absorbing panel 54 is used to cover the lightning cable 14 and one of the highly reflective shear webs 15 of the spar structure 13.

These modifications will now be described in more detail below.

Referring still to Figure 4, the shell 11, 12 of the blade 50 comprises regions of sandwich panel construction. The principles of sandwich panel construction have already been discussed by way of background and so will not be repeated here. Four sandwich panel regions are shown in Figure 4: a first pair of sandwich panels 56 (hereinafter the leading edge panels') is incorporated respectively in the upper and lower shells 11, 12 between the leading edge 21 of the blade and the spar structure 13 and a second pair of panels 58 (hereinafter the trailing edge panels') is incorporated respectively in the upper and lower shells 11, 12 between the spar structure 13 and the trailing edge 16 of the blade 50.

In this example, the trailing edge panels 58 are adapted to transmit radar signals 52a, 52b, whilst the leading edge panels 56 are adapted to absorb I attenuate radar signals 52c in the same way as described above by way of background in relation to Figures 2 and 3. This is because the electrical thicknesses of the leading edge panels 56 are not close to half the wavelength of the incident radar signals 52a, 52b and are hence suitable for single-core RAM sandwich panels, whereas the trailing edge panels 58 are thicker, with a thickness approaching half the wavelength of the incident radar signals 52a, 52b, and so split core RAM panels would normally be used here. It will be appreciated that the present invention provides an alternative to split core RAM panels.

Referring to Figure 4A, which shows an enlarged schematic view of part of a trailing edge sandwich panel 58, the panel 58 comprises a foam core 60 disposed between inner and outer GRP skins 62 and 64 respectively. The outer GRP skin 64 of the sandwich panel 58 comprises a frequency selective surface (FSS) 66 in the form of a patterned array of conductive material, which is printed on a layer of glass fabric using highly conductive silver-based ink. The FSS layer 66 is included in the laminate layup of the shell 11 when the shell 11 is manufactured! which results in the FSS layer 66 becoming embedded in the outer GRP skin 64. The sandwich panel 58 of the lower shell 12 has an identical structure. As an alternative, the FSS layer 66 may be embedded in the inner skin 62. This arrangement advantageously minimises the risk of lightning strikes.

In this example, the ESS 66 is tuned to transmit radar signals of approximately 3 GHz, which results in the sandwich panels 58 being substantially transparent to radar signals of this frequency. However, it will be appreciated that the FSS 66 may be suitably tuned to transmit any desired frequency.

Inside the cavity 37 of the blade 50, the radar absorbing panel 54 is bonded to a surface of the shear web 15 and also covers the lightning cable 14, which is attached to the shear web 15. The panel 54 is made mainly from lightweight foam or any suitable material. A circuit analogue (CA) layer 68 is provided on a first surface 70 of the panel 54 facing the trailing edge 16 of the blade 50. The circuit analogue layer 68 is printed on the first surface 70 using carbon-based ink. A reflective ground plane 72 in the form of a layer of carbon veil is provided on a second surface 74 of the panel facing the leading edge 21 of the blade 50. In other embodiments, the ground plane 72 may comprise a printed reflector layer or a ESS polymer layer.

When the blade 50 is in use, radar signals 52a and 52b are transmitted by the modified trailing edge panels 58 into the cavity 37. Inside the cavity 37, the radar signal 52a is incident upon the shear web 15 and the lightning cable 14 and is absorbed by the radar-absorbing panel 54, which covers these components. The radar signal 52b is not incident upon these components and so passes straight through both shells 11, 12 of the blade 50. Some internal reflection of the signal 52b may occur from the inner skin 19, however these reflected signals should be largely absorbed by the RAM inside the cavity 37. At the leading edge 21 of the blade 50, the leading edge panels 56 largely attenuate incident radar signals 52c instead of transmitting those signals.

Referring now to Figure 5, which shows a wind turbine blade 75 in accordance with a second embodiment of the present invention. In this embodiment, a smaller radar-absorbing foam panel 76 is used to cover just the lightning cable 14. This smaller panel 76 is omega-shaped in cross section, which is a convenient shape because the round part 77 of the omega profile covers the lightning cable 14 whilst the legs 78 of the omega profile provide a convenient surface for bonding the panel 76 to the shear web 15.

A CA pattern 80 is printed on a first surface 82 of the panel 76, which generally faces the trailing edge 16 of the blade 75. In this example, the lightning cable 14 itself forms a reflective ground plane for the radar absorbing panel 76, which avoids the need to provide a separate conductive ground plane such as a layer of carbon veil. This advantageously avoids the risk of arcing, which can occur when conductive material is placed near to the lightning cable 14.

In an alternative embodiment, the omega panel 76 may be made from plastic to which radar absorbing filler particles are added. These filler particles may include carbon nanotubes and other carbon particulates, ferrites and lossy dielectrics The thickness of the profile and the type and concentration of the filler particles are all chosen to produce RAM performance at the chosen frequency range.

The RCS of the shear web 15 in this embodiment is reduced by incorporating radar absorbing filler particles in the composite structure of the shear web 15. These particles may include carbon nanotubes and other carbon particulates, ferrites and lossy dielectrics, and are included in the resin used to manufacture the shear web 15.

Alternatively, a circuit analogue method may be used to adapt the shear web.

The trailing edge sandwich panels 58 in the second embodiment are modified to transmit radar signals in the same way as those of the first embodiment described above with reference to Figure 4.

Various modifications may be made to the examples described above without departing from the scope of the present invention as defined by the accompanying claims. For example, whilst a wind turbine blade has been described by way of example, it will be appreciated that the principles of the present invention may be applied to other wind turbine components such as the hub, the nacelle or the tower for example. Also, whilst a shear web and a lightning cable have been described above by way of example, it will be appreciated that any other reflective components inside the cavity may be suitably adapted to absorb radar signals, such as via the application of RAM panels to their surfaces or by including RAM within their structure to make them intrinsically radar absorbing. Further, whilst the blades described above by way of example have RAM sandwich panels at the leading edge that are designed to absorb radar signals, in other embodiments the leading edge panels may have a similar construction to the trailing edge panels, i.e. constructed to optimise the transmission of radar signals.

Claims

Claims 1. A wind turbine component comprising an outer shell defining an internal cavity, wherein at least a part of the outer shell is adapted to transmit incident radar signals into the cavity, and one or more reflective components inside the cavity are adapted to absorb radar signals.
2. The wind turbine component of Claim 1, wherein the outer shell includes a frequency selective surface that is suitably tuned to transmit incident radar signals into the cavity.
3. The wind turbine component of Claim 2, wherein the frequency selective surface is embedded within the outer shell.
4. The wind turbine component of any preceding claim, wherein radar absorbing material is applied to the or each reflective component inside the cavity.
5. The wind turbine component of Claim 4, wherein the radar absorbing material comprises a radar absorbing panel.
6. The wind turbine component of Claim 5, wherein the reflective component acts as the ground plane for the radar absorbing panel.
7. The wind turbine component of Claim 6, wherein the reflective component is a metallic component of a lightning protection system.
8. The wind turbine component of Claim 6 or Claim 7, wherein the reflective component is a cable that is attached to a surface, and the panel is omega-shaped in cross section, with the round part of the omega profile covering the cable and the legs of the omega being bonded to the surface.
9. The wind turbine component of any preceding claim, wherein at least one of the reflective components is of composite construction and comprises radar absorbing filler particles within its composite structure.
10. The wind turbine component of any preceding claim! wherein the adapted part of the outer shell is of sandwich panel construction and comprises a layer of core material, which is disposed between inner and outer skin layers.
11. The wind turbine component of any preceding claim, wherein the wind turbine component is a wind turbine blade.
12. A wind turbine comprising the wind turbine component of any preceding claim.
13. A wind farm comprising the wind turbine of Claim 12.
14. A method of reducing the radar cross section of a wind turbine component, the component comprising an outer shell defining an internal cavity, and the method comprising optimising the transmission of incident radar signals through at least pad of the outer shell and into the cavity and optimising the absorption of the radar signals inside the cavity.
15. A wind turbine blade optimised according to the method of Claim 14.