Wind farm wake mapping from SAR
Merete B. Christiansen(1) and Charlotte B. Hasager(1)
Risoe National Laboratory,
DK - 4000 Roskilde,
Offshore wind farming is progressing rapidly in Europe. Knowledge about the wind farm wake effect (i.e. the reduction of wind speed caused by wind turbines) is valuable in environmental impact studies. Moreover, it is useful in the planning of new wind farms. Construction of wind farms in clusters is attractive because maintenance costs and grid connections can be shared. At the same time, the distance between wind farms must be sufficient to avoid shadowing. The optimum spacing of wind farms within clusters is not yet known. Imaging radars provide spatial information, which is very valuable for quantification of wind farm wake effects. In the following, analyses of wind farm wake effects from synthetic aperture radar (SAR) are presented.
The two largest offshore wind farms in the world are located at the Danish sites Horns Rev and Nysted. The wind farm at Horns Rev became operational in late 2002 with 80 turbines and a total capacity 160 MW. The wind farm at Nysted became operational in mid-2003 with 72 turbines and a total capacity 166 MW. At both sites, the total turbine height is 110 m above mean sea level. Micro-siting and environmental impact studies are currently in progress at Horns Rev and Nysted, as the development of two additional wind farms is scheduled in the near-vicinity of the existing farms.
A series of satellite and airborne SAR images were analyzed to determine the downstream distance over which the wind farms at Horns Rev and Nysted impact the marine wind climate. The satellite data were high-resolution ERS-2 SAR and ENVISAT ASAR images. Airborne E-SAR scenes were acquired over Horns Rev on October 12, 2003 by the German Aerospace Center (DLR). The E-SAR data had a much higher spatial resolution (2 m) than the satellite SAR images (25 m). A disadvantage was the longer acquisition time (2-4 minutes per scene) that allowed the wind speed and direction to fluctuate within a single E-SAR scene. Wind maps were generated from the SAR images through inversion of the geophysical model function CMOD4. Spatial averages of wind speed were obtained upstream, within, and downstream of the wind turbine arrays. Velocity deficits up to 20% of the ambient wind speed were found downstream of the wind farms. Wake effects were observed for downstream distances of 5-20 km, depending on the ambient wind speed, the atmospheric stability and the fraction of turbines operating during SAR data acquisitions.