Wind turbine power and sound in relation to atmospheric stability

Atmospheric stability cannot, with respect to modem, toll 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 near-ground 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. Copyright (C) 2007 John Wiley & Sons, Ltd.