1 Meteorology, Wind Energy Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark2 Wind Energy Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark3 Risø National Laboratory for Sustainable Energy, Technical University of Denmark4 Department of Wind Energy, Technical University of Denmark
Synthetic aperture radars (SAR), mounted on satellites or aircraft, have proven useful for ocean wind mapping. Wind speeds at the height 10 m may be retrieved from measurements of radar backscatter using empirical model functions. The resulting windfields are valuable in offshore wind energy planning as a supplement to on site measurements, which are costly and sparse, and model wind fields, which are not fully validated. Two applications of SAR measurements in offshore wind energy planning areaddressed here: the study of wind farm wake effects and the potential of using SAR winds in offshore wind resource assessment. Firstly, wind wakes behind two large offshore wind farms in Denmark – Horns Rev and Nysted – are identified. A region ofreduced wind speed is found downstream of both wind farms from the SAR wind fields. The wake extent and magnitude depends on the wind speed, the atmospheric stability, and the fraction of turbines operating. Wind farm wake effects are detected up to 20 kmdownwind of the last turbine. This distance is longer than predicted by state-of-the art wake models. Wake losses are typically 10-20% near the wind farms. Secondly, the potential of using SAR wind maps in offshore wind resource assessment isinvestigated. The resource assessment is made through Weibull fitting to frequency observations of wind speed and requires at least 100 satellite observations per year for a given site of interest. Predictions of the energy density are very sensitive tothe wind speed and the highest possible accuracy on SAR wind retrievals is therefore sought. A 1.1 m s-1 deviation on the mean wind speed is found through comparison with mast measurements at Horns Rev. The accuracy on mean wind speeds and energydensities found from satellite measurements varies with different empirical model functions. Additional uncertainties are introduced by the infrequent satellite sampling at fixed times of the day. The accuracy on satellite based wind resource assessmentis sufficient in the early stage of wind farm planning, before higher-accuracy on site measurements are available.