The purpose of the project is to give an empirical description of the noise caused by rotor details: design of blade-tips, trailing edge thickness and slits at emergency brakes.
In order to be able to design wind turbine rotors with low aerodynamic noise emission it is necessary to determine parameters essential to the generation of aerodynamic noise, to implement prediction models and thereby to develop low noise tip designs for a full scale field test.
A computer code for the prediction of tip noise has been implemented. For simple tip shapes it can give some guidelines to a low noise design, although no detailed relationship between physical dimensions of the tip and the tip vortex description, necessary as input to the prediction model, has been established.
Several different tip designs have been manufactured and evaluated empirically on the selected test turbine. A preliminary analysis of the results show that the A weighted sound power level at reference operating conditions deviated approximately 7 dB as the tips are changed. The produced electrical power seems not be influenced.
Noise from wind turbines is a significant matter in wind turbine siting. In Europe many potential wind turbine locations can only be exploited by low noise wind turbines. The aerodynamic rotor noise is the dominant noise source from usual wind turbines. Other sources with origin in the machinery components can be reduced with common noise control technology. With a reduction of the rotor noise the wind energy potential can be utilized to a further extent.
From earlier experience it was found, that significant parameters in aerodynamic noise emission from wind turbine rotors are tip design and trailing edge noise. The project is on this basis initiated with a literature study noise prediction models are developed. Based upon the rotor noise models the influence of the most significant rotor design parameters are investigated, and concepts for design of low noise rotors are drawn up. Two tip designs are selected from the spectrum of low noise designs for further investigations.
At this stage of the project the prediction models and the two selected designs are discussed with the other EC project group. From the discussions and also an evaluation by aerodynamic noise experts of the Stuttgart University (JOUR-CT90-0111) the tip designs will eventually be modified and improved.
The measurements will be carried out on a commercial grid connected wind turbine of 200-500 kW. The wind turbine will be specially selected for an insignificant mechanical noise level. The measurements will be performed in the wind speed range from 5-12 m/s. Measurements of aerodynamic noise and electric power output will carried out simultaneously.
Measurements will be carried out on the two different low noise tip designs attached and fitted to the existing rotor blades. Other measurements will include modifications of the trailing edge. Furthermore, measurements of noise from different sizes of slits at the emergency brakes will be performed.