Radar absorbing material compatible with lightning protection systems
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.
1. 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.
2. The wind turbine component of claim 1 , wherein the ground plane comprises a ferroelectric material.
3. The wind turbine component of claim 2 , wherein the ferroelectric material is selected from barium titanate (BaTiO 3 ), lead titanate (PbTiO 3 ), sodium nitrite (NaNO 2 ) and lead zirconate titanate (PZT).
4. The wind turbine component of claim 1 , wherein the ground plane comprises a ferrimagnetic material.
5. The wind turbine component of claim 4 , wherein the ferrimagnetic material is selected from magnetite; a hexaferrite or other ferrites composed of iron oxides and other elements.
6. The wind turbine component of claim 1 , wherein the ground plane comprises a percolating material combination.
7. The wind turbine component of claim 6 , wherein the percolating material combination comprises particles of conductive material dispersed in a non-metallic host.
8. The wind turbine component of claim 7 , wherein the non-metallic host comprises a polymer matrix.
9. The wind turbine component of claim 7 , wherein the conductive particles are particles of metal or carbon.
10. The wind turbine component of claim 1 , wherein the ground plane is optimised to reflect radar signals having a frequency in the range of 1-10 GHz.
11. The wind turbine component of claim 1 , wherein the ground plane has a sheet resistance of less than 0.02 Ω/sq in the presence of an electric field having a frequency of 1 GHz and above.
12. The wind turbine component of claim 1 , wherein the ground plane has a dielectric constant of 80-120 in the presence of an electric field having a frequency of 1 GHz and above.
13. The wind turbine component of claim 1 , wherein the ground plane has a dielectric constant of 90-110 in the presence of an electric field having a frequency of 1 GHz and above.
14. The wind turbine component of claim 1 , wherein the ground plane has a dielectric constant of 95-105 in the presence of an electric field having a frequency of 1 GHz and above.
15. The wind turbine component of claim 1 , wherein the ground plane has a sheet resistance that is greater than 100,000 Ω/sq in the presence of an electric field having a frequency of 10 MHz and below.
16. The wind turbine component of claim 1 , wherein the ground plane has a dielectric constant of 1-10 in the presence of an electric field having a frequency of 10 MHz and below.
17. The wind turbine component of claim 1 , wherein the ground plane has a dielectric constant of 2-6 in the presence of an electric field having a frequency of 10 MHz and below.
18. The wind turbine component of claim 1 , wherein the ground plane has a dielectric constant of 3-5 in the presence of an electric field having a frequency of 10 MHz and below.
19. The wind turbine component of claim 1 , wherein the component includes at least part of a lightning protection system.
20. The wind turbine component of claim 1 , wherein the component includes one or more lightning receptors.
21. The wind turbine component of claim 20 , wherein the ground plane abuts the one or more lightning receptors.
22. The wind turbine component of claim 1 , wherein the radar absorbing material further comprises an impedance layer spaced apart from the ground plane.
23. The wind turbine component of claim 22 , wherein the impedance layer is a circuit analogue layer.
24. The wind turbine component of claim 22 , wherein the radar absorbing material further comprises at least one dielectric layer between the impedance layer and the ground plane.
25. The wind turbine component of claim 1 , wherein the wind turbine component is a rotor blade.
26. A wind turbine comprising the component of claim 1 .
27. A wind farm including the wind turbine of claim 26 .
28. 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.