Accounting for the speed shear in wind turbine power performance measurement

Abstract

Abstract:The power curve of a wind turbine is the primary characteristic of the machine as it is the basis of the warranty for it power production. The current IEC standard for power performance measurement only requires the measurement of the wind speed at hub height and the air density to characterise the wind field in front of the turbine. However, with the growing size of the turbine rotors during the last years, the effect of the variations of the wind speed within the swept rotor area, and therefore of the power output, cannot be ignored any longer. Primary effects on the power performance are from the vertical wind shear and the turbulence intensity. The work presented in this thesis consists of the description and the investigation of a simple method to account for the wind speed shear in the power performance measurement. Ignoring this effect was shown to result in a power curve dependant on the shear condition, therefore on the season and the site. It was then proposed to use an equivalent wind speed accounting for the whole speed profile in front of the turbine. The method was first tested with aerodynamic simulations of a multi-megawatt wind turbine which demonstrated the decrease of the scatter in the power curve. A power curve defined in terms of this equivalent wind speed would be less dependant on the shear than the standard power curve. The equivalent wind speed method was then experimentally validated with lidar measurements. Two equivalent wind speed definitions were considered both resulting in the reduction of the scatter in the power curve. As a lidar wind profiler can measure the wind speed at several heights within the rotor span, the wind speed profile is described with more accuracy than with the power law model. The equivalent wind speed derived from measurements, including at least one measurement above hub height, resulted in a smaller scatter in the power curve than the equivalent wind speed derived from profiles extrapolated from measurements at hub height and below only. It is well established that the turbulence intensity also influences the power performance of a wind turbine. Two ways of accounting for the turbulence were tested with the experimental data: an adaptation of the equivalent wind speed so that it also accounts for the turbulence intensity and the combination of the equivalent wind speed accounting for the wind shear only with the turbulence normalising method for turbulence intensity suggested by Albers. The second method was found to be more suitable for normalising the power curve for the turbulence intensity. Using the equivalent wind speed accounting for the wind shear in the power performance measurement was shown to result in a more repeatable power curve than the standard power curve and hence, in a better annual energy production estimation. Furthermore, the decrease of the scatter in the power curve corresponds to a decrease of the category A uncertainty in power, resulting in a smaller uncertainty in estimated AEP.The thesis is submitted to the Danish Tec...

Ten Years of Boundary-Layer and Wind-Power Meteorology at Høvsøre, Denmark

“…6.1) are observed mainly from the western sector in spring and autumn when the difference between the land and sea temperatures is relatively large. A wind speed representative of the whole profile, a weighted-average wind speed over the swept rotor area, was found to significantly reduce the sensitivity of the power curve to vertical wind shear (Wagner et al 2011). This was accomplished using a ground-based lidar.…”

“…First, the effect of vertical wind shear on wind-turbine power curves was demonstrated based on aeroelastic simulations and mast measurements ) and turbine and lidar measurements (Wagner et al 2011). The site experiences a range of wind-speed profiles for different directions, due to large thermal effects for easterly winds and the influence of the abrupt change of roughness for westerlies.…”

A study of stability effects in forested terrain

“…the equivalent wind speed for AEP method [18] and the cosine loss model for inflow angle [19]. However, these methods are highly technology dependant as some turbines are now being designed with the ability to adjust their operation to mitigate losses due to such effects [20] …”

Generic Methodology for Field Calibration of Nacelle-Based Wind Lidars

“…The effects of wind speed and direction variations -called shear and veer respectively -over the rotor swept area on power curves can no longer be neglected [1]. Measuring the wind at one point, e.g.…”