Radar’s Impact on Wind Farms Studied

The British Wind Energy Association (BWEA) is offering up the results of a study that provides a detailed understanding of the interactions between wind farms and radar systems. The commissioning of the study aims to counter wind farm opposition from an issue that has little or no scientific evidence.

London, England – October 17, 2003 [SolarAccess.com] The process of obtaining planning permission to build a wind farm involves many considerations, including consultation with various aviation stakeholders. These parties may raise objections for a variety of reasons, with a known source of objections being that the wind farm may appear on the display of air traffic control radar, said BWEA. The organization said decisions made regarding the likely impact that a wind farm may have upon radar operations are currently based upon assumptions. The electromagnetic interactions between a wind turbine and a radar signal are complex and there is currently limited understanding in this area and no accepted method for quantifying this potential impact. A conflict of interest currently exists between the desire to encourage wind farm development as a renewable energy source and the desire to maintain the operational safety of air traffic, said BWEA. The practical manifestation of this conflict is that the UK has seen objections against a significant proportion of proposed wind farms on the grounds of aviation safety. The DTI established a “Wind Frms, Civil Aviation and Defense Interests Working Group” to tackle these issues. As part of the activity of the working group, UK-based QinetiQ were commissioned to undertake the study, which was entirely funded by the UK’s Department of Trade and Industry’s (DTI) Sustainable Energy Program. The main objectives were as follows: – To determine the effects of siting wind turbines adjacent to primary air traffic control radar. – To determine the extent to which detailed design of wind turbines influences their effects on radar systems. – To determine the extent to which the design of the radar processing influences the effects of wind turbines on radar systems. – To provide text suitable for inclusion on the UK guidelines on Wind Energy and Aviation Interests. SUMMARY This study has focused on the development and validation of a computer model that can be used to predict the radar reflection characteristics (Radar Cross Section, which is measured in square meters and is normally presented on a logarithmic scale) of wind turbines and understand the complex interaction between radar energy and turbines. The scope of the model includes: – The affects of the radar propagation over the terrain between the radar and the wind farm. – The dynamic radar scattering from the wind turbines. – The signal processing in the radar. – Display of results via a simulated radar display. The model was validated through a full-scale trial, using a QinetiQ mobile radar system to collect data for a single operational wind turbine at Swaffham in Norfolk. The model was then used to perform a detailed sensitivity analysis and to compile a list of the key factors influencing the radar signature of wind turbines. CONCLUSIONS The results from the project have enabled BWEA to provide a much more detailed quantification of the complex interactions between wind turbines and radar systems than was previously available. The key turbine factors influencing the effect of a wind turbine on radar are: – The design of the tower and nacelle should have the smallest RCS signature possible. -RCS of the tower and nacelle can be effectively reduced though careful shaping. – Large turbines do not necessarily lead to large RCS (i.e. tower height does not greatly affect RCS. – Blade RCS returns can only be effectively controlled though the use of absorbing materials. – A low probability of detection, but a large clutter return can be expected when the yaw angle of the turbines is close to 90 degrees from the radar direction. – A high probability of detection, but a smaller area of clutter, can be expected when the yaw angle of the turbines is close to 0 degrees and 180 degree from radar direction. The key factors influencing the effect of wind farms on radar are: – Spacing of wind turbines within a wind farm needs to be considered in the context of the radar cross range/down range resolutions. Spacing the turbines such that only one turbine can appear in any range cell has advantages in identifying the wind farm, filtering out the turbines and in tracking aircraft over the farm area. – In a circumstance where a single wind turbine in clear line of sight to the radar is undetected, it is highly likely that a wind farm of similar wind turbines would also be undetectable. – No optimal layout or format has been prescribed, as each wind farm will have its own specific requirements dependent on many factors. Key terrain and shadowing factors include: – In non line of sight situations, the level of detectability of the wind farm is dependent on the frequency of radar and the distance from the wind farm to the point of diffraction and the distance below the line of sight horizon, where the wind farm is located. – Single wind turbines do not create a significant “radar shadow.” Any shadow region is only dark to a distance of a few hundred metres behind the turbine. – Beyond this there is some reduction of the radar power, and a time-variation, but these will not prevent detection except possibly for very small targets.

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