1 Test and Measurements, Wind Energy Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark2 Wind Energy Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark3 Risø National Laboratory for Sustainable Energy, Technical University of Denmark4 Meteorology, Wind Energy Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark5 Department of Wind Energy, Technical University of Denmark
LIDAR systems are getting more and more accurate and reliable. It has been shown many times that the mean horizontal wind speed measured by a lidar over flat terrain compares very well with that measured by a cup anemometer. But can a lidar measure turbulence? Here we investigate the case of a continuous wave, conically scanning Zephir lidar. First, the wind speed standard deviation measured by such a lidar gives on average 80% of the standard deviation measured by a cup anemometer. This difference is due to the spatial averaging inherently made by a cw conically scanning lidar. The spatial averaging is done in two steps: 1) the weighted averaging of the wind speed in the probe volume of the laser beam; 2) the averaging of the wind speeds occurring on the circular path described by the conically scanning lidar. Therefore the standard deviation measured by a lidar resolves only the turbulence structures larger than a length scale depending on the circle diameter and the mean wind speed (range of magnitude: 100m). However, the Zephir lidar gives another turbulence quantity, the so-called turbulence parameter, which can resolve turbulence structures with a smaller length scale. In this paper, we suggest a volumetric filtering of the turbulence to represent the effect of the spatial averaging operated by a lidar when measuring the wind speed. We then evaluate this model by comparing the theoretical results to experimental data obtained with several Zephir systems, for both turbulence quantities.
Wind energy; Test and measurements; Risø-R-1682; Risø-R-1682(EN); Test og målinger; Vindenergi
Main Research Area:
Denmark. Forskningscenter Risoe. Risoe-r
Danmarks Tekniske Universitet, Risø Nationallaboratoriet for Bæredygtig Energi, 2009