Wake effects of large offshore wind farms identified from satellite SAR

The effect of large offshore wind farms on the local wind climate is studied using satellite synthetic aperture radar (SAR). Wind maps are derived from a total of 30 ERS-2 SAR scenes and 11 Envisat ASAR scenes over two large offshore wind farms in Denmark: Horns Rev in the North Sea (80 turbines) and Nysted in the Baltic Sea (78 turbines). The wind farms are the world's largest to date. For the first time, high-resolution SAR-derived wind speed images are utilized to identify regions of reduced wind speed and high turbulence intensity (i.e. wind wakes) downstream of wind turbine arrays. After quality control, 19 SAR scenes are available for the study of wake effects. A decrease of the mean wind speed is found as the wind flows through the wind farms, leaving a velocity deficit of 8-9% on average, immediately downstream of the wind turbine arrays. From this point, wind speed recovers to within 2% of the free stream velocity over a distance of 5-20 km depending on the ambient wind speed, the atmospheric stability and the number of turbines in operation. The wake magnitude and extent found from SAR wind maps is consistent with in situ measurements and results from wake models. The standard deviation of SAR-derived wind speeds is an indicator of turbulence intensity. Added turbulence intensity downstream of a wind farm is found for 7 of the 19 cases. Turbulence intensity is not as easily identified from SAR images as changes in the mean wind speed. This may be due to a low impact of wind turbine generated turbulence on the sea surface where SAR measurements are obtained.