LiDAR (Light Detection And Ranging) measurements have become very popular in the last years. Because its compact dimensions and the capability to measure up to 200 m and higher with great precision more and more measurements are conducted with LiDARs. Do we still need met mast based measurements?
Most projects do significantly benefit from using a LiDAR for site assessment because of its advantages compared to met masts and pressure of time within the project schedules. When looking into detail on the method of measurement, some points have to be kept in mind for application.
LiDAR measures wind speed by determining the speed of dust particles (aerosol) on different locations above the LiDAR. Compared to the rotating cups of an anemometer the weight of the particles is negligible small. Therefore, measurements taken during gusts or slowdowns of the wind speed will be measured different compared to cup anemometers.
Caused by the cone angle of usually 28 - 30° (the angle of each beam to the vertical), the distance between single measurement points increases with measurement height. Wind conditions are different on each measurement point and also taken measurement values are different compared to single point cup anemometer measurements.
For this reason, verification is required to determine the measurement uncertainty of the LiDAR using calibrated cup anemometers. During the verification test, the LiDAR is placed next to a high, IEC61400-12-1 Ed.2 compliant reference mast and the simultaneously measured data are evaluated according to valid guidelines. The verification should be performed at the hub height of the planned wind turbine.
LiDAR devices are increasingly used as the only measuring device and must then measure for at least 6 months at the site of the planned wind farm. In some scenarios, for example in complex terrain, it is nevertheless not always possible to do without a measuring mast. In any case, a LiDAR measurement at sites in the planned wind farm area improves the data base, so that a reduction of the uncertainty of energy yield assessment and an increase of P75 and P90 can be achieved.
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