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of the rotor effective wind speed v0(t) is a weighted average over all available vi available
during time t (Figure 114(right)). That means that the five focus distances are shifted to one
single distance. This shift can be done by the use of Taylor’s frozen turbulence hypothesis
which is valid for horizontally Lidar measurements (Schlipf et al., 2010).
Figure 114: Circle trajectory used for the CART-2 campaign (left) and schematic drawing of
the incoming wind field measured by a lidar at different focus distances (right).
In (Figure 115(left)) three different wind speeds are shown in a 10-min time series: one is
measured by the anemometer on the met mast vM, the other one shows the rotor effective
wind speed v0L measured with the Lidar. The third wind speed is recalculated from the
turbine data as e.g. the pitch angle, rotor speed and power output data. Here the turbine
represents a big horizontal axes anemometer with a rotor effective wind speed v0. It is visible
that the turbine’s wind speed corresponds much better to the rotor effective wind speed of
the Lidar than to the anemometer on the met mast.In a further step, the power production
of the CART-2 turbine was simulated with FAST. As input for the wind two different data
sets were used: the real met mast data and the rotor effective wind speed measured with
the Lidar. In (Figure 115(right)) the result of the simulated power output is shown together
with the real electrical power from the turbine. As one can see, the simulation with the rotor
effective wind speed corresponds very well with the real data (Rettenmeier et al., 2012b).
Figure 115: Time series of the rotor effective wind speed from Lidar and turbine and of the
anemometer mounted on a mast (left) and simulation of the power production with met mast
and Lidar data in comparison with the real production data. (right).
8.9 Outlook & Conclusions
The use of a lidar system from the nacelle offers various applications in wake wind field
analysis, wind turbine control, power curve determination and load estimation. The whole
swept rotor area can be taken into account, but assumptions have to be made. Nacelle-based
measurement methods show a great potential on- and offshore and even in complex terrain.
But there are still some investigations that have to be done concerning turbulence, vertical
and horizontal shear, comparison criteria and equivalent wind speed.
168 DTU Wind Energy-E-Report-0029(EN)