Testing the Performance of a Ground-based Wind LiDAR System

Traditional meteorological masts that are commonly used in wind energy applications are limited and very expensive to install in offshore locations. Therefore, it is necessary to search for an alternative method to replace the standard wind measurements (cup/sonic anemometers, vanes) used on a mast. The LiDAR (Light Detection And Ranging) technique has become a reasonable alternative in the last years with the advantage that higher vertical resolution measurements over the whole rotor diameter of a wind turbine can be performed [1], [2]. However, before taking lidar systems as an accepted wind measurement alternative, we have to validate as well as understand how this technique computes both mean and turbulent wind parameters. One important issue is the time resolution at which the wind measurements have to be sampled which will depend mostly on the application for which the wind measurements will be used. For instance, for wind resource and power curve assessments, a 10-min interval is recommended by the standards [3], [4]; however several investigations pointed out that a higher resolution (1 Hz or even less) is needed for a better definition of the power curve [5]. In addition, monitoring the rapid changes in the wind conditions is important for optimal operation of wind farms. In the last years, several field experiment intercomparisons with in-situ sensors located on meteorological masts have proven the lidar system to be a reliable instrument for obtaining mean values (10-min). In particular, similar lidar systems as the one used in this study (Leosphere WindCube) have been tested over homogeneous terrain (e.g., [6], [7]) with excellent results. However such comparisons have only relied on 10-min averaged data. Within the joint research project RAVE (Research at AlphaVEntus), the RAVE-LIDAR work package deals with remote sensing measurements using lidar systems at the German Offshore test site [8]. Part of the work carried out by DEWI is presented in this article. This study aims to examine the performance of a wind lidar measurement located at FINO1 site, using the mast to provide reference measurements. The present study is split into two major parts. In the first part, an intercomparison between lidar and conventional mast-based instruments, such as cup/sonic and vanes, is performed based on 10-min data. Several wind parameters are considered, namely wind speed, wind direction and turbulence intensity. In the second part, we complement the 10-min data intercomparison with a spectral analysis in order to get a more complete picture of the flow in the ranges between one hour and about one second. B. Cañadillas, A. Westerhellweg, T. Neumann; DEWI GmbH, Wilhelmshaven