Publishers version - DTU Orbit

7 What can remote sensing contribute
to power curve measurements?
Rozenn Wagner
DTU Wind Energy, Risø Campus, Roskilde, Denmark
7.1 Introduction
Power performance measurement is central to the wind industry since it forms the basis for
the power production warranty of the wind turbine. The power curve measurement has to
be realised according to the IEC 61400-12-1 standard. The power curve is obtained with
10-min mean power output from the turbine plotted against simultaneous 10-min average
wind speeds. The standard requires the wind speed to be measured by a cup anemometer
mountedontop ofamast havingthe same heightasthe turbine huband locatedat a distance
equivalent to 2.5 rotor diameters from the turbine.
Such a plot usuallyshowsasignificantspread ofvalues and not a uniquelydefined function.
The origin of the scatter can mainly be grouped into three categories: the wind turbine components
characteristics, sensor error and the wind characteristics. Within the last group, the
current standard only requires the wind speed at hub height and the air density measurement.
However, other wind characteristics can influence the power production like the variation of
the wind speed with height (i.e. wind speed shear). The influence of wind speed shear on
the power performance was shown in several studies: some based on aerodynamic simulations
(Antoniou, 2009; Wagner et al., 2009) others based on measurements (Elliot and Cadogan,
1990; Sumner an Masson, 2006).
A major issue is to experimentally evaluate the wind speed shear. The wind speed profile
is usually assumed to follow one of the standard models such as the logarithmic or power law
profiles. However, these models are valid for some particular meteorological conditions, and
therefore, cannot represent all the profiles experienced by a wind turbine. Measurements are
then a better option but are also challenging. Indeed characterising the wind speed profile
in front of the rotor of a multi-MW wind turbine requires measurements of wind speed at
several heights, including some above hub height, i.e. typically above 100 m. Remote sensing
instruments such as lidar or sodar then appear as a very attractive solution.
This chapter starts with a description of the influence of the wind speed shear on the power
performance of a multi-MW turbine. The challenge of describing the wind speed profile is
then discussed followedby a description of an experiment using a lidar for its characterisation.
This is followed by the introduction of the definition of an equivalent wind speed taking the
wind shear into account resulting in an improvement of the power performance measurement.
Finally, some recommendations about remote sensing instruments are given to successfully
apply this method.
7.2 Power performance and wind shear
7.2.1 Shear and aerodynamics
In order to see the effect of the wind speed shear on a wind turbine, aerodynamic simulations
were carried out for two inflow cases:
1. constant wind speed profile (same wind speed everywhere) with 8 m s −1
2. power law profile with 8 m s −1 at hub height and a shear exponent of 0.5
DTU Wind Energy-E-Report-0029(EN) 143