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Denmark. The turbine was a 500 kW Nordtank turbine with a rotor diameter of 41 m and a
hub height of 36 m. A meteorological mast was situated at a distance of approximately 2.2
rotor diameters, so absolute as well as relative measurements could be carried out. The met
mast was equipped withcup anemometersat heights of18,27, 36,45 and 54mabove ground
level, spanning the entire swept area of the rotor. Figure 55. below shows a comparison of
hub height wind speed measured by the lidar (at a range of 2.2 rotor diameters) and the met
mast, when the wind direction was in a sector that was unaffected by shadowing or wake
effects from adjacent turbines. It can be seen that the measurements demonstrate a high
level of correlation, particular when the sloping nature of the terrain at this site is taken into
account.
Figure55:CorrelationplotofhubheightwindspeedsmeasuredbyanacellemountedCWlidar
and a pair of cup anenometers on a 500 kW Nordtank wind turbine at Roskilde, Denmark.
The lidar data was then reprocessed to measure wind speed and direction at the height of
each of the cups listed above, taking full account of any nacelle motion on both measurement
height and line-of-sight velocity. Turbine SCADA power output data was corrected for air
density and temperature and used to generate rotor equivalent power curves from both the
metmastandthelidardata(Figure56).Thispreliminaryresultindicatesthatturbinemounted
CW lidars are capable of measuring power curves of comparable quality to met masts, but
with the advantage of easy redeployment. These are believed to be the first reported rotor
equivalent power curve measurements using a commercially available nacelle mounted lidar.
Subjects of current research in the turbine mounted lidar area include feed-forward control
for stress load reduction and short range (i.e. less than 2 rotor diameters) wind speed measurement
for turbines situated in more complex terrain where long range measurements are
less representative of the actual wind field incident on the turbine rotor.
4.9 Summary, state of the art, and future developments
Coherent monostatic CW lidar is a method capable of rapid wind speed measurement at
relatively short ranges (all the way from 10 m to 200 m) and hence is well suited to several
96 DTU Wind Energy-E-Report-0029(EN)