Wind turbine wake measurements with automatically adjusting scanning trajectories in a multi-Doppler lidar setup
Abstract. In the context of the Perdigao 2017 experiment, the German Aerospace
Center (DLR) deployed three long-range scanning Doppler lidars with the dedicated
purpose of investigating the wake of a single wind turbine at the
experimental site. A novel method was tested for the first time to
investigate wake properties with ground-based lidars over a wide range of
wind directions. For this method, the three lidars, which were space- and
time-synchronized using the WindScanner software, were programmed to measure
with crossing beams at individual points up to 10 rotor diameters downstream
of the wind turbine. Every half hour, the measurement points were adapted to the
current wind direction to obtain a high availability of wake measurements in
changing wind conditions. The linearly independent radial velocities where
the lidar beams intersect allow the calculation of the wind vector at those
points. Two approaches to estimating the prevailing wind direction were tested
throughout the campaign. In the first approach, velocity azimuth display (VAD) scans of one of the
lidars were used to calculate a 5 min average of wind speed and wind
direction every half hour, whereas later in the experiment 5 min
averages of sonic anemometer measurements of a meteorological mast close to
the wind turbine became available in real time and were used for the scanning
adjustment. Results of wind speed deficit measurements are presented for two
measurement days with varying northwesterly winds, and it is evaluated how
well the lidar beam intersection points match the actual wake location. The
new method allowed wake measurements to be obtained over the whole measurement
period, whereas a static scanning setup would only have captured short
periods of wake occurrences.