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When radars meet wind turbines

Wind turbines disturb the operation of radars, especially weather radars. "If a wind turbine is located less then 30 kilometers from a weather radar, the wind speed measurement by the radar, based on the Doppler* effect, can be altered," explains Gérard Bobillot, engineer at Onera. "The degree of the disturbance depends on the size of the wind turbine, and how many there are. But that’s not the only problem: forecasts of rain along the beam axis can also be disturbed because of a masking effect.".

Under these circumstances, it’s easier to understand why three quarters of all refusals for wind turbine installation permits are due to the presence of weather radars. However, this also provides an opportunity for Onera to bring its expertise to the fore, by modeling these disturbances to better understand them, and then finding ways of resolving the problem.

Onera has in fact built up very solid experience in determining radar cross-sections. Manufacturers of wind turbines and wind farm project developers have therefore asked Onera to carry out this type of evaluation for their construction permits.

Faced with this recurring problem, the French Ministry of Ecology, Energy, Sustainable Development and the Sea decided to launch a research program, coordinated by the French environment and energy management agency Ademe. The program is divided into three phases, with the first starting operation in 2010 :

• Onera and Oktal SE will develop Sipré, a simulation application (including the Fermat software program) for radar disturbances caused by wind turbines, covering an area of several dozen square kilometers. At the end of 2010, these simulations will be compared to measurements made in the field (weather radar, wind farms). Using this application, engineers can simulate planned installations and provide a scientific basis for discussions between weather services and project developers.
• Two other phases are planned as well. Wind turbine blades could be made "stealthier" (less detectable by radar), an area of expertise at Onera, by using radar-absorbent materials in the blades; in fact, EADS Astrium has already built a concept demonstrator. At the same time, radar manufacturers are developing signal treatment methods (also an area of expertise at Onera) to compensate for the effects of wind turbines.

* The Doppler effect is the change in frequency of a wave for an observer moving relative to the source of the wave. In a radar system, measuring this change enables calculating speed.