Wind turbine wake measurements with automatically adjusting scanning trajectories in a multi-Doppler lidar setup

Norman Wildmann,Thomas Gerz,Nikola Vasiljevic

doi:10.5194/amt-11-3801-2018

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 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.

Highlights

In state-of-the-art wind-energy research, the investigation of the flow field downstream of a wind turbine – i.e., in its wake – is currently a topic of high relevance for the siting and operation of turbines, especially in a typical configuration of a wind farm with multiple collocated turbines
In Vasiljevic (2014) it is explained in detail how hard-target calibration of the pointing direction can be done, and how the timing synchronization based on a precise GPS clock and synchronization commands of the master computer is realized in the WindScanner system
The results show that there is a large spread in the measured wind speed deficits

Summary

Introduction

In state-of-the-art wind-energy research, the investigation of the flow field downstream of a wind turbine – i.e., in its wake – is currently a topic of high relevance for the siting and operation of turbines, especially in a typical configuration of a wind farm with multiple collocated turbines. For this purpose it is important to understand how high static and dynamic loads of neighboring turbines will be. While different measurement systems have been used – such as radars (Hirth et al, 2012), sodars (Barthelmie et al, 2003) and small remotely piloted aircraft (RPA; Wildmann et al, 2014) – it is lidar technology of different categories (pulsed and continuous wave) that has proven to be the most versatile tool for performing wake measurements because of the high availability and reliability, good resolution and flexible possibilities to probe the atmosphere especially with scanning systems

Methods

Results

Conclusion