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10
5
Power 5 ms
Power 6 ms
Power 7 ms
Power 8 ms
Power 9 ms
O Power 10 ms
KE
0.2 0.1 0.1 0.2 0.3 0.4 0.5
5
10
Figure 90: Normalised difference in power output and kinetic energy flux between shear and
uniform case as function of the shear exponent, for various wind speed at hub height
These results clearly indicate that wind speed profiles encountered by the turbine during a
power performance measurement should be known and taken into account.
7.3 Wind speed profiles
Within the power performance evaluation context, the wind speed shear is often described
by α obtained from the assumption of a power law profile (Eq. (159)). This procedure was
applied to profiles measured by a high meteorological mast located at the test site for large
wind turbines, at Høvsøre, on the west coast of Denmark with zhub = 80 m and z = 40 m
to determine α. For some cases, this model fits the measured profile very well, but it cannot
represent all kinds of profiles observed at Høvsøre. Figure 91 shows two examples of measured
profiles and their corresponding modelled profiles.
The distribution of the error made by such an approximation is shown in Figure 92. We
define the error as the difference between the wind speed at 116.5 m (top of the mast)
estimated by the power law model and the speed measured by the cup anemometer. Over a
year of measurements, for a large wind sector 60 ◦ –300 ◦ degrees (with predominant wind from
west), 7% of the profiles show a wind speed error at 116.5 m larger than 10%. We should
keep in mind that all the anemometers are mounted on a boom except the top anemometer,
and this can induce an error in the profile extrapolation to the top (116.5 m).
As shownby the simulationspresented in section7.2, such an errorin the windspeed profile
can significantly affect the power curve. The shear exponent from wind speed measurements
at two heights is not acceptable for this application. Therefore it is important to measure the
wind speeds at several heights below and above hub height. For this purpose remote sensing
instruments such as lidar and sodar are highly relevant since in many cases they can measure
up to 200 m with the required degree of accuracy.
An experimental campaign using a lidar to measure the wind speed profile in front of
a multi-MW turbine showed the importance of measuring the complete profile for power
performance. In our experiment, the lidar measured the wind speed at 9 uniformly distributed
heights covering 90% of the vertical rotor diameter. Each wind speed profile measured by the
lidar was fitted to a power law profile in order to find the most representative shear exponent
for this profile (αfit). The fit is forced through the point of coordinate (uhub, zhub):
αfit ufit(z) = uhub
z
zhub
. (163)
In order to separate the profiles for which the power law assumption was acceptable, we
148 DTU Wind Energy-E-Report-0029(EN)
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