Presentations

Abstract

Dual-Doppler scanning lidars offer the possibility to measure wakes at scales ranging from a few hundred meters to several kilometers. They can therefore facilitate comprehensive coverage of atmospheric dynamics in and around wind farms at several highly relevant scales ranging from the individual turbine to cluster wakes. High fidelity wake measurements with scanning lidars can significantly support the efforts towards high fidelity wake modelling by using the data as validation. This work demonstrates offshore dual-Doppler wake measurements at different scales, performed with a control software that allows to synchronise and adapt the scan pattern to the observed wind conditions. Two scan strategies are evaluated: a dual PPI (plan position indicator) scan to capture fine scale dynamics within the wind farm and a stepstare campaign to measure the spatial evolution of the wake at larger scales behind the wind farm. An analysis of the deviations in measurement height due to the inclined PPI planes and a thrust-induced turbine tilt was carried out and showed that caution is advised when interpreting dual PPI data. Nevertheless, the dual PPI scans showed to be an effective tool in capturing the spatial characteristics of wakes within the wind farm on the scale of a single turbine or a few neighbouring turbines. Step-stare scans on the other hand have the advantage of lower uncertainties due to less height deviations, but the spatial resolution is limited. The results of the step-stare measurement were compared to widely used engineering wake models (TurbOPark and Jensen) and showed a good agreement regarding the trends, but the models overestimated the deficits by around five to eight percentage points. The control software to dynamically adapt the scanning patterns to the observed wind conditions ensured a highly efficient and tailored measurement design that optimised measurement time in the area of interest while increasing the sampling rate.