Noise emission from wind turbines in wake

When installing wind turbines iin clusters or wind farms, the inflow conditions to the wind turbines can be distrubed due to wake effects from other wind turbines. The effect of wake on noise generation from wind turbines is investigated. The work is based on measurements carried out on a 2 MW wind turbine. To investigate the relationship between the far field noise levels and the surface pressure and inflow angles measured by sensors on an instrumented wind turbine blade, a parabolic measurement system was designed and tested. Based on the measurements it is concluded that: - The variance of surface pressure at the trailing edge (TE) agrees with the theory with regard to variation of pressure spectra with varying inflow angle (AoA) to the blade. Low frequency TE surface pressure increases with increased AoA and high frequency surface pressure decreases with increased AoA; - It seems that the TE surface pressure remains almost unaltered during wake operation; - Results from the surface transducers at the leading edge (LE) and the inflow angles determined from the pitot tube indicates that the inflow at LE is more turbulent in wake for the same AoA and with a low frequency characteristic, thereby giving rise to more low frequency noise generated during wake operation; - The far field measurements supports that on one hand there will be produced relative more low frequency noise due to a turbulent indlow to the blade, and on the other hand there will be produced less noise in the broader frequency range/high frequency range due to a lower inflow angle caused by the wind deficit in the wake. The net effect of wake on the total noise level is unresolved. A study of wake effects for differnet layout of wind farms due to the general wind speed reduction in wake has been made using the WindPRO Nord2000 program. The basic assumption for this study has been that a turbine operating in wake radiates the same noise as a turbine with free inflow at the same power output. It is seen that the largest reduction of noise at the receptors is when the closest turbines are in a deep wake. For different wind farm layout it is concluded that the combined wake influence on the noise impact is a complex function of wind direction, wind speed and wind farm geometry. It may cause deviations from the undisturbed noise impact of several dB, and the effect may change rapidly with small changes in wind direction