Modelling Wind Farm Noise
Mr Mark Simpson
Noise Emissions from Wind Turbines
The noise emissions from wind turbines are a function of wind speed and wind shear. For the purposes of this analysis the wind shear coefficient is taken the wind tip (at closest approach to the ground) and the turbine hub height. This represents the maximum likely wind speed change over the lower half of the wind turbine. Typically, wind shear is a function of the upwind terrain, the hub height, length of wind turbine blade. Wind shear is typically independent of wind speed.
For real world sites the wind shear is usually determined by measurements. In a paper Wind Shear, Taller Turbines, and the Effects on Wind Farm Development create a need for Taller MET Towers by Livingstone and Anderson in 2004 it was found that the wind shear coefficient is often much higher than would normally be determined from analysis of the wind shear at lower heights (normally where wind shear is greatest). This implies that the turbine is exposed to higher wind velocities than is allowed at design time. The wind shear coefficient may be greater than 0.2 in the region of the wind turbine hub.
The PENV311 model allows for a wind speed dependent noise profile to be entered into the model. The stall speed of the wind turbine is noted along with the directional wind-shear coefficient and the low frequency noise emissions. The model also allows for a single low frequency emission to be entered into the model, typically about 5 Hz to 10 Hz, The NASA 1990 technical paper Wind Turbine Acoustics by Hubbard and Shepherd[140] identifies that the noise spectrum for wind turbines peak at a very low frequency and has described this as the low frequency rotational noise of the wind turbine. This noise is described as impulsive and is potentially a greater issue with downwind horizontal axis wind turbines than with upwind horizontal axis wind turbines.
The PENV3 noise model also allows the directivity of the noise emissions to be entered into the model. Two directivity patterns are entered, one for the low frequency rotational noise and the other for the blade broadband noise. Generally it is found that the highest noise levels occur upwind and downwind of the wind turbine. Perpendicular to the direction of the wind the noise may be lower, typically 10dB for broad band noise, and perhaps 15dB or more for low frequency rotational noise...
11 PEN3 is a revision of PEN3D referenced in this Paper for the Turitea wind farm Hearing.
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