Study of the interaction of atmospheric wakes from several offshore wind farms as observed by Synthetic Aperture Radar (SAR) system

Offshore wind turbines induce atmospheric wakes downstream of typically tens of kilometers length while converting wind energy into electrical power. In the recent years, wakes behind single wind farms have been extensively studied. The growing number of offshore wind farms (OWFs) in the German Bight increases the occurrence of interferences between close neighboring OWFs. Previous studies showed that the proximity of neighboring OWFs impacts the performance of downwind turbines and reduces their capacity factor. However, the interactions of wakes from different wind farms is not well understood, in particular concerning the resulting wind speed deficits, turbulence intensities and superimposed wake lengths.

The interaction of wakes from two OWFs generally leads to longer wakes. Also, it was observed frequently that  wakes resulting from 2 OWFs reach a third wind farm. Several conditions can entail interactions of wakes from several OWFs, such as the size of OWF (geometry and density of turbines), atmospheric stability and wind speed and directions. The study focuses on the investigation of the interactions of wakes from two and three OWFs using Synthetic Aperture Radar system (SAR), which is an interesting instrument to observe large spatial areas at resolution of 20 m.  The statistical analysis of a 5-year period of SAR data acquisitions by the  Sentinel-1A and Sentinel1-B satellites revealed that the occurrence of interactions of wakes between OWFs exceeds 75% of all the cases for which wakes were observed. The interaction between OWFs is clearly correlated with a certain wind direction range. Additionally, the geometry of OWFs plays a role in the two-dimensional structure of the wakes and the potential for impacting neighboring wind farms. Obviously, wind directions parallel to the alignment of several OWFs are likely to induce strong interactions of wakes.

SAR data are combined with stability information from atmospheric models and mast measurements to analyse the respective impacts on key parameters of superimposed wakes (e.g. deficit, wake length).  Results are compared with simplified empirical models, which make assumptions about the linearity of the wake superposition process.