Abstract
Offshore wind farm cluster effects between neighboring wind farms increase rapidly with the large-scale deployment of offshore wind turbines. The wind farm wakes observed from Synthetic Aperture Radar (SAR) are sometimes visible and atmospheric and wake models are here shown to convincingly reproduce the observed very long wind farm wakes. The present study mainly focuses on wind farm wake climatology based on Envisat ASAR. The available SAR data archive covering the large offshore wind farms at Horns Rev has been used for geo-located wind farm wake studies. However, the results are difficult to interpret due to mainly three issues: the limited number of samples per wind directional sector, the coastal wind speed gradient, and oceanic bathymetry effects in the SAR retrievals. A new methodology is developed and presented. This method overcomes effectively the first issue and in most cases, but not always, the second. In the new method all wind field maps are rotated such that the wind is always coming from the same relative direction. By applying the new method to the SAR wind maps, mesoscale and microscale model wake aggregated wind-fields results are compared. The SAR-based findings strongly support the model results at Horns Rev 1.
Highlights
In the Northern European Seas offshore wind farms are planned as clusters
The wind farm wake climatology can be modelled by micro- and mesoscale models but perfect agreement cannot be expected between Synthetic Aperture Radar (SAR) and wake model results
The SAR wind time series at 10 m is used as input, the winds are extrapolated to 70 m and the wake is modeled at that height and the winds are extrapolated downwards to 10 m again
Summary
In the Northern European Seas offshore wind farms are planned as clusters. The wind farm wake from one wind farm has the potential to influence the power production at neighboring wind farms. In order to show that wind farm wakes are detectable from SAR, we present in this study one case based on RADARSAT-2 ScanSAR Wide This scene is a good example where ideal conditions for wake analysis occur and the coverage is just right for capturing 10 large offshore wind farms located in the southern North Sea. We compare the instantaneous SAR-based wind farm wakes to micro- and mesoscale wake model results. The wind farm wake climatology can be modelled by micro- and mesoscale models but perfect agreement cannot be expected between SAR and wake model results This is due to the different nature of data with SAR based on the sea surface while wake models operate around wind turbine hub-height.
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