The effect on noise emission from wind turbines due to ice accretion on rotor blades

Abstract: Swedish EPA (Naturvårdsverket) noise level guide-lines suggest that equivalent A-weighted sound pressure levels (SPL) must not exceed 40 dBA at residents. Thus, in the planning of new wind farms and their location it is crucial to estimate the disturbance it may cause to nearby residents. Wind turbine noise emission levels are guaranteed by the wind turbine manufacturer only under ice-free conditions. Thus, ice accretion on wind turbine may lead to increased wind turbine noise resulting in noise levels at nearby residents to exceed 40 dBA SPL. The purpose of the project is to evaluate the effect on wind turbine noise emission due to ice accretion. This, by trying to quantify the ice accretion on rotor blades and correlate it to any change in noise emission. A literature study shows that the rotor blades are to be considered the primary noise source. Hence, ice accretion on rotor blades are assumed to be the main influence on noise character. A field study is performed in two parts; as a long term measurement based on the method out-lined by IEC 61400-11 and as a short term measurement in strict accordance with IEC 61400-11. These aim to obtain noise emission levels for the case of icing conditions and ice-free conditions (reference conditions) as well as background noise levels. An analysis is performed, which sets out to correlate ice measurements with wind turbine performance and noise emission. Data reduction procedures are performed according to IEC 61400-11.The apparent sound power levels are evaluated. This is performed for the case of icing conditions as well as for the case of ice-free onditions. A statistical evaluation of icing event is carried out. The results show that ice accretion on wind turbine (rotor blades) may lead to drastically higher noise emission levels. The sound power levels show an average increase of 10.6 dB at 8 m/s. However, this can occur at all wind speeds from 6 m/s to 10 m/s. Higher levels of noise, (55 to 65 dBA SPL) may be caused by very small amounts of ice accretion. Occurrences of higher levels of noise, in the range of 50 to 65 dBA SPL, are not common. Noise levels exceeding 50 dBA SPL are to expected 10.3 % of the time during the winter or 3 % of the time during one year. Correlation between measured ice accumulation and noise level is weak apart from large amounts of ice. This due to statistical noise. Taking into account the noise level guide-lines of 40 dBA SPL at residents, as is recommended by Swedish EPA (Naturvårdsverket), the increased levels of windturbine noise under icing conditions may force the power production to a halt.