Wind Turbine Wake Measurements with Automatically Adjusting Scanning Trajectories in a Multi-Doppler Lidar Setup

Abstract

Abstract. In the context of the Perdigão 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 established 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 ten rotor diameters downstream the wind turbine. Every half hour, the 5 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 estimate the prevailing wind direction were tested throughout the campaign. In the first approach, VAD scans of one of the lidars were used to calculate a five-minute average of wind speed and wind direction every half hour, whereas later in the experiment, five-minute averages of sonic anemometer measurements of a meteorological 10 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 westerly winds and it is evaluated how well the lidar beam intersection points match the actual wake location. The new method allowed to obtain wake measurements over the whole measurement period, whereas a static scanning setup would only have captured short periods of wake occurrences. The analysed cases reveal that state-of-the-art engineering models for wakes underestimate the actual wind speed deficit.

Automated Wind Turbine Wake Characterization in Complex Terrain

“…• RHI scans can provide vertical slices through the wake as it moves downstream but if incorrectly aligned will produce 'empty' scans i.e. scans of the wind field without wakes unless wake tracking is employed (Wildmann et al, 2018).…”

“…estimation of direction and wind speed at a height equal to the wind turbine to be determined and provide a vertical profile of wind speeds within the valley for use in other research. Minor adjustments were made to the scan pattern in February 2017 to improve the efficiency of the scan including reducing the number of range gates to 41 making the maximum distance scanned 1230 m. In May 2017 the lowest elevation angle of 7° was removed because it was frequently blocked by vegetation and additional RHI scans directly towards the wind turbine at ~ 227° (Az: 226.9 and 227.4°) were added for comparison with other lidar data sets from the 5 IOP ( Barthelmie et al, 2018;Wildmann et al, 2018). These modifications were kept to a minimum to ensure continuity in the subsequent data analysis.…”

On the self-similarity of wind turbine wakes in complex terrain using large-eddy simulation

“…Simulations are performed on a domain spanning over 5120×2560×3000 m 3 , which represents a double ridge configured site named Perdigão in Portugal . The site has been a focus of intensive field measurement campaign under the Perdigão experiment 2017 (Wildmann et al, 2018; and the New European Wind Atlas (NEWA) project (Mann et al, 2017). Figure 1 shows the overview of the site, where, the wind direction is along the main transect (235 degrees relative to North).…”

Analysis of flow in complex terrain using multi-Doppler lidar retrievals

“…Higher levels of uncertainty have been found with more complicated scan strategies (Choukulkar et al, 2017). For multi-Doppler retrievals, the magnitude of uncertainty was directly related to the ability to precisely coordinate scans (Wildmann et al, 2018b). Additionally, higher levels of uncertainty are present during unstable conditions (Newman et al, 2016).…”