High Quality Test Stand for Commercial Nacelle-Lidar Accredited Calibrations
Per Vølund, Cecilie Langhans, Torkel Dyrkorn Løland, Flemming Langhans
COWI A/S, 2800 Lyngby, Denmark
In the village Tårs at the West Coast of the Danish Island Lolland a test-site for calibration of nacelle-lidars has been established.
The test-site consists of a stiff mast with two platforms for simultanous mounting of two Nacelle-Lidars which are to be calibrated. 300 meter towards south west 3 lattice masts of 20 m height have been installed. Each mast has a top anemometer, a wind vane and at lower height a sonic-anemometer. A central logger collects simultaneous data from the masts and the Nacelle-Lidars.
With a careful installation the Nacelle-Lidar beams simultanously will hit each one of the two outer top anemometers. Within less than an hour a lidar can be adjusted to ensure that the two beams both hit the anemometers at the same time.
The quality of measurements is high because of a number of developments. The position at a west coast ensures high frequency of high wind speeds and low turbulence. The height of 20m ensures limited turbulence - compared to test-sites with masts up to 10m. The use of three masts ensures that a central wind measurement can be used for control of the wind speed analysis result from the Nacelle-Lidar. The use of sonics some meter below the top anemometers ensure control of vertical wind profile.
Calibrations can be carried out during a short time frame - in practice 4 weeks for the first test of two Nacelle-Lidars. The high quality and sampling rate to catch full data-range are important improvements for Nacelle-Lidar users. Several Nacelle-Lidars for offshore use have been calibrated.
Recommendations of Courtney, DTU, in the report "Calibrating Nacelle-Lidars" are implemented and the system is improved significantly through several steps.
A site at a windy west coast ensures high wind speeds with low turbulence. Test height of 20m above ground ensures limited turbulence compared to other test sites with mast heights of less than 10m. The use of two simultaneous masts for line-of-sight calibration reduces measurement period to half. The use of a third mast in the middle between the two line-of-sight masts enables control of the Lidar analysis wind results. The use of wind vane and sonic below the top anemometer ensures recording of vertical wind profile.
The method has been reviewed by DONG and Leosphere, and used for several 2-beam Wind Iris.
Calibrations at the new test facility can be carried out during much shorter time - in practice 4 weeks for the first test of two nacelle-lidars. The high quality of data and quick sampling rate to catch full data range are important improvements for nacelle-lidar users. Costs of calibrations are simultaneously cut drastically.
Nacelle-Lidars are becoming standard for power curve evaluations in especially in offshore wind farms. They must be based on high quality calibrations of the Nacelle-Lidars. Such calibrations must be provided at low cost and with short lead time - which the presented test facility on a commercial basis offers to the business.
The use of Nacelle-Lidars are becoming state of the art in power curve evaluation for offshore wind farms. To ensure high quality of the measurements traceable calibrations of the nacelle-lidars is a must.
The presented test facility reduces significantly the time needed from delivery of a Nacelle-lidar from factory to installation offshore after calibration. 2 beams are calibrated simultanously. 2 nacelle-lidars are calibrated simultaneously. Measurement height of 20 m and location on a windy west coast ensures fast sampling of sufficient high wind speed data.