SAR for offshore wind energy

Description

Offshore wind farms are increasingly common in coastal waters. They experience gradients in the wind resources across the wind farm that need to be quantified before construction. In operation, they extract energy from the wind reducing the downstream wind speed (wake) and thus affect surrounding wind farms. Both effects can be modelled but lack large scale high resolution reference data to check and validate the models. SAR based wind retrievals offer such measurements.
Large archives from past missions like Envisat or Radarsat-1 are available from a time before most offshore wind farms have been built. These can serve as a baseline of wind conditions before wind farms are present for most cases. New data from the current satellites Sentinel-1A/B let the archives grow fast. Wind maps from these sensors are processed using the Geophysical Model Function CMOD5.N and are publically available under https://satwinds.windenergy.dtu.dk/.
Offshore wind farm development is emerging at the US East Coast and wind resource atlases from models have been created. We inter-calibrated wind maps from the above mentioned SAR sensors and created a SAR satellite based wind atlas. Comparison with the modelled data revealed differences in mean wind speed gradients in the coastal zone. Sites for future wind farms are close to shore, mostly between 10 and 50 km. Wind resources within these potential sites vary by less than 0.1 m s-1 from models but around 0.3 m s-1 from SAR. Spatial variation of wind resources are important for correctly estimating the yield from a wind farm and optimize its placement. Due to a higher resolution, SAR derived wind atlases are sensitive to smaller scale variations that are not present in the model.
Sentinel-1 images show the presence of wind farm wakes downstream of offshore wind turbines. Derived wind maps show a wind speed reduction in this area. For the first time, the wake detected in SAR imagery was compared to wind fields from Doppler radars around the wind farm Westermost Rough in the UK. We could confirm that the structure of the wake can be observed from SAR on scales of 150 m. At the Anholt wind farm in Denmark, wakes were characterized by comparing wind conditions in SAR images before and after construction of the wind farm. Evidence of wind farm wakes was found even for conditions where strong wind speed gradients are present.
With an increasing archive of SAR data that becomes increasingly open for wind energy purposes, the data are becoming more reliable and cheaper. SAR derived wind atlases can be useful, especially in areas where offshore wind farms are emerging. Applications of wake detection are also becoming more useful as Sentinel-1 data is free to use and available all over Europe.

Period	13 May 2019 → 18 May 2019
Event title	European Space Agency's 2019 Living Planet Symposium
Event type	Conference
Location	Milan, Italy
Degree of Recognition	International