To identify the influence of wind shear and turbulence on wind turbine performance, flat terrain wind profiles are analysed up to a height of 160 m. The profiles' shapes are found to extend from no shear to high wind shear, and on many occasions, local maxima within the profiles are also observed. Assuming a certain turbine hub height, the profiles with hub-height wind speeds between 6 m s −1 and 8 m s −1 are normalized at 7 m s −1 and grouped to a number of mean shear profiles. The energy in the profiles varies considerably for the same hub-height wind speed. These profiles are then used as input to a Blade Element Momentum model that simulates the Siemens 3.6 MW wind turbine. The analysis is carried out as time series simulations where the electrical power is the primary characterization parameter. The results of the simulations indicate that wind speed measurements at different heights over the swept rotor area would allow the determination of the electrical power as a function of an 'equivalent wind speed' where wind shear and turbulence intensity are taken into account. Electrical power is found to correlate significantly better to the equivalent wind speed than to the single point hub-height wind speed. Figure 8. (a): Calculated power curves with fi xed and variable rotational speed. No shear; (b): Calculated power curves with fi xed and variable speed. Extreme shear specifi ed. 246 × 76 mm (500 × 500 DPI) 356 R. Wagner et al. ConclusionsLarge variations have been observed in the wind profi les over a fl at site. The results show that the profi les usually do not follow the logarithmic law; instead, their shape, in the case of fl at terrain, heavily depends upon the atmospheric conditions. These profi les were used as input to a sensitivity analysis concerning the power production as a function of a weighted ('equivalent') wind speed over the whole swept rotor area as compared with the results of the wind speed only at hub height. The results from the simulations indicate that measuring the wind speed at an increased number of points over the whole swept rotor area profi le, would improve the correlation between wind input and power output. These results support the necessity for the introduction of a new defi nition for power performance measurements using a distributed measurement of the wind over the swept rotor area instead of using only the hub-height wind speed.

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...

Methods to measure the vertical flux of horizontal momentum using both continuous wave and pulsed Doppler lidar profilers are evaluated. The lidar measurements are compared to momentum flux observations performed with sonic anemometers over flat terrain at Høvsøre, Denmark, and profile-derived vertical momentum flux observations at the Horns Rev wind farm in the North Sea. Generally, the momentum fluxes are reduced because of the finite measuring volume of the instruments, and the filtering is crudely accounted for theoretically. The essential parameter for the estimation of the reduction is the ratio of the turbulence scale to the size of the measuring volume. For the continuous wave lidar the reduction can largely be compensated by averaging Doppler spectra instead of radial velocities.

Operational since 2004, the National Centre for Wind Turbines at Høvsøre, Denmark has become a reference research site for wind-power meteorology. In this study, we review the site, its instrumentation, observations, and main research programs. The programs comprise activities on, inter alia, remote sensing, where measurements from lidars have been compared extensively with those from traditional instrumentation on masts. In addition, with regard to wind-power meteorology, wind-resource methodologies for wind climate extrapolation have been evaluated and improved. Further, special attention has been given to research on boundary-layer flow, where parametrizations of the length scale and wind profile have been developed and evaluated. Atmospheric turbulence studies are continuously conducted at Høvsøre, where spectral tensor models have been evaluated and extended to account for atmospheric stability, and experiments using microscale and mesoscale numerical modelling.

Enevoldsen, K. (2009).Comparison of 3D turbulence measurements using three staring wind lidars and a sonic anemometer. Meteorologische Zeitschrift, 18(2), 135-140. DOI: 10.1127/0941-2948/2009/0370 Meteorologische Zeitschrift, Vol. 18, No. 2, 135-140 (April 2009 Open Access Article AbstractThe goals are to compare lidar volume averaged wind measurement with point measurement reference sensors and to demonstrate the feasibility of performing 3D turbulence measurements with lidars. For that purpose three pulsed lidars were used in staring mode, placed so that their beams crossed close to a 3D sonic anemometer mounted at 78 m above the ground. The results show generally very good correlation between the lidar and the sonic times series, except that the variance of the velocity measured by the lidar is attenuated due to spatial filtering. The amount of attenuation can however be predicted theoretically by use of a spectral tensor model of the atmospheric surface-layer turbulence. ZusammenfassungDas Ziel ist es, volumengemittelte LiDAR-Windmessungen mit Punktmessungen von Referenzsensoren zu vergleichen sowie die Möglichkeit aufzuzeigen, 3D-Turbulenzmessungen mit LiDAR-Geräten durchzuführen. Zu diesem Zweck wurden drei gepulste LiDAR-Systeme mit fixer Blickrichtung so aufgestellt, dass ihre Strahlen nahe eines 3D-Ultraschall-Anemometers kreuzten, welches 78 müber Grund befestigt war. Die Ergebnisse zeigen im Allgemeinen sehr gute Korrelationen zwischen den Zeitreihen der LiDAR-und Ultraschall-Anemometer, allerdings wird die Streuung der vom LiDAR gemessenen Geschwindigkeit durch räumliches Filtern abgeschwächt. Der Grad der Abschwächung kann jedoch mittels eines spektralen Tensormodells, das die Turbulenz in der atmosphärischen Bodenschicht beschreibt, theoretisch vorausberechnet werden.