Wind turbine power and sound in relation to atmospheric stability

Abstract

Atmospheric stability cannot, with respect to modern, tall wind turbines, be viewed as a 'small perturbation to a basic neutral state' . This can be demonstrated by comparison of measured wind velocity at the height of the rotor with the wind velocity expected in a neutral or 'standard' atmosphere. Atmospheric stability has a significant effect on wind shear and increases the power production substantially relative to a neutral atmosphere. This conclusion from Dutch data is corroborated by other published wind shear data from the temperate climate zone. The increase in wind shear due to atmospheric stability also has a significant effect on the sound emission, causing it to be substantially higher than predicted from near-ground wind velocity and a neutral atmosphere, resulting in a higher noise impact on neighbouring residences. Several measures are proposed to mitigate the noise impact. To reduce noise levels, the rotational speed can be controlled with the nearground wind speed or sound level as the control input. To reduce the fluctuation in the sound ('blade thumping'), it is suggested to adjust the blade pitch angle of the rotating blades continuously. To prevent stronger fluctuations at night due to the coincidence of thumps from several turbines, it is suggested to add random variations in pitch angle, mimicking the effect of large-scale turbulent fluctuations in daytime. Figure 1. Solid lines: 1987 wind velocity averaged per clock half hour at heights (bottom to top) of 10 to 200 m; dotted line: logarithmically extrapolated V 80 ; +: shear exponent m 10,80 Wind Turbine Power and Sound 155 Figure 4. Distribution of shear exponent per meteorological season, determined from V 80 /V 10 Figure 5. Shear exponent m from wind velocity gradient between 10 and 80 m (left), and 40 and 140 m (right) versus total ground heat flow; grey circles: all data, black dots: V 80 > 4 m s −1 158 G. P. van den Berg Figure 6. Wind direction change between 10 and 80 m (left) and 40 and 140 m (right) versus shear exponent m between same heights for V 80 > 4 m s −1 Figure 7. Prevalence of shear exponent m between 10 and 80 m (top) and 40 and 140 m (bottom) in four seasons and year of 1987 160 G. P. van den Berg Values of wind shear have been reported by various authors, showing similar results. Pérez et al. 4 measured wind velocities up to 500 m above an 840 m altitude plateau north of Valladolid, Spain, for every hour over 16 months.The shear exponent, calculated from the wind velocity at 40 and 220 m, varied from 0·05 to 0·95, but was most of the time between 0·1 and 0·7. High shear exponents occurred more often than in Cabauw: m > 0·48 for 50% of the time. This is likely the result of the more southern position: insolation is higher, causing 162 G. P. van den Berg

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