Improvement of power curve measurement with lidar wind speed profiles

Introduction Power performance measurement is central to the wind industry since it forms the basis for the power production warranty of the wind turbine. For that reason it should be independent of the wind characteristics. A wind turbine power performance measurement consists of measuring simultaneous wind speed in front of the turbine and power output of the turbine. Ten minutes averages of these parameters are used to generate the power curve (power as a function of the wind speed) and the power coefficient (Cp, the ratio between the turbine power output and the kinetic energy flux). The IEC 61400-12-1 standard for wind turbine power performance measurement [1] only requires measurements of the wind speed at hub height and the air density (derived from temperature and pressure measurements) to characterise the wind field surrounding the wind turbine. However it has been shown that other wind characteristics such as the variation of the wind speed with altitude (wind speed shear) and the fast variation of wind speed around the 10 minutes mean wind speed (turbulence) can also influence the power performance of a large turbine [2]. A few studies, focusing on the effect of the wind speed shear, showed that the power production decreased with increasing shear, [3], [4]. The assumption that the profile is constant over the rotor swept area (so that the wind speed at hub height is representative for the whole area) leads to inconsistencies in power curve measurements, [5] and [6]. It is therefore important to characterize the wind speed profile in front of the swept rotor area. As a simple extrapolation from below to above hub height may result in an erroneous estimation of the kinetic energy, it is preferable to actually measure the wind speed over the entire rotor disc. In this paper, we describe an experiment in which wind speed profiles were measured in front of a multi-megawatt turbine using a lidar. Those measurements were used to apply the equivalent wind speed method suggested in [7].