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ECN publication
Flow analysis with nacelle-mounted LiDAR
Published by: Publication date:
ECN Wind Energy 14-10-2016
ECN report number: Document type:
ECN-E--16-041 ECN publication
Number of pages: Full text:
38 Download PDF  

In the LAWINE project, several LiDAR systems are evaluated, among which Wind Iris from Avent Lidar Technology. This LiDAR is installed on the nacelle of a 2.5MW research wind turbine at the ECN test site EWTW. Two measurement campaigns have been organised with this LiDAR system: one as a forward looking LiDAR system to measure the horizontal wind speed and direction ahead of the turbine, and one as a backward looking LiDAR system to measure the wake of the turbine. This report presents results from the analysis of the Wind Iris in forward and backward looking orientation. In forward looking orientation, the objective of this study is to identify possible disturbing factors when measuring the incoming wind speed in front of the rotor. In backward looking orientation, the objective is to quantify the accuracy of wake measurements with a single beam LiDAR system.

The results in forward looking orientation have shown that the blockage effect of the turbine is noticeable up to a distance of 3.5D in front of the rotor. Blockage effects of other wind turbines disturb the measurements in a much wider sector than expected from plots of measured turbulence intensities. When one of the lines of sight passes the rotor of another turbine within a distance of 2.5D from the rotor centre, the measurement of the Wind Iris is disturbed by the flow around that turbine. For accurate measurement of the undisturbed wind speed, it is therefore recommended to measure the wind speed at a distance of 3.5D from the rotor, while the lines of sight at this distance are at least 2.5D separated from the rotor axis of nearby turbines.

In backward looking orientation, measurements of the velocities at the wake centreline agree very well with FarmFlow calculations with differences of approximately 2% for single wake conditions. Also for double wake conditions the agreements are quite good. However, wake effects behind the third turbine seem to be underestimated by the Wind Iris, probably due to measurement uncertainties of the wind direction and yaw angle. To reduce these uncertainties in the future, it is necessary to measure along different lines of sight, for example with a scanning LiDAR, so that it will be possible to determine the real position of the wake centreline.

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