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Wind turbines emit noise and therefore have to be kept at a distance from residential buildings.
© marikond / shutterstock
Flow movements along rotor blades
08.03.2016

Wind turbines are becoming quieter

Wind turbines are not allowed to disturb neighbouring residential buildings through the noise that they emit. Wind turbines therefore have to be kept at a distance from settlements. To further reduce and better predict noise emissions, the University of Siegen has now launched a research project. In this project, the scientists want to modify the trailing edges of the rotor blades. They also want to develop better sound forecasts for entire wind farms. Quieter wind turbines would enable more land to become available for wind power generation.

Noise appraisals play an important role as part of the approval procedures for wind farms. As part of these appraisals it is calculated whether the limits of 35 dB (A) at night and 50 dB (A) during the day can be met in neighbouring residential areas once the wind turbines begin operation. Even though wind turbine manufacturers have managed to considerably reduce noise emissions in recent years through design changes and technical control measures, the possibilities are still far from exhausted. The University of Siegen is therefore working with the Senvion wind turbine manufacturer to develop a low-noise rotor blade. The scientists in Siegen, who have extensive experience in the flow acoustics field, are investigating how the rotor blade trailing edge can be structurally improved to reduce noise emissions. Professor Thomas Carolus, project manager at the University of Siegen, explains: “We hope that our improved blade will reduce noise emissions by 1 to 3 dB (A). At best this would enable the noise emissions to be halved relative to today's systems.” The second subject area of the research project is concerned with improving the noise forecasts for entire wind farms. Here, the noise emissions of individual wind turbines combine to create an overall noise level.

The trailing edge is decisive

With rotor blades, part of the wind flows above and below the blade profile. Ultimately, this creates the force that drives the rotor. The air flow, however, becomes very turbulent as it moves along the blade surface. This therefore generates noise along the trailing edge of the blade where the two streams meet again. This source of noise is one of the few places where significant improvements are still possible in modern wind turbines. By altering the flow pattern along the blade’s trailing edge and geometric modifications, the intention is to shift the noise-producing turbulence away from the edge, which reduces the noise generation.

The scientists in Siegen are developing several blade segment models on a reduced scale and are testing them in their own wind tunnel. The segment with the best results will be built at full scale and tested in a large, aero-acoustic wind tunnel. The results of the Siegen research will be incorporated in Senvion’s blade design. Carolus: “A low-noise blade opens up interesting economic prospects for using wind energy. Quieter wind turbines would also enable usage in some areas currently kept free of wind farms for noise abatement reasons. In addition, some of the wind turbines located in noise-sensitive areas have had their maximum power restricted in order to reduce the noise.”

Forecasting wind farms

It is difficult to predict the noise emitted by proposed wind farms during operation. The rotational effects of the blades, their deformation in accordance with the wind loads and the respective operating point of the wind turbines affect the sound generated along the blades. The current meteorological conditions, such as the wind direction, can also increase or decrease the propagation of the sound in certain areas. In order to predict the noise emitted by wind farms, the scientists in Siegen are therefore developing a new calculation program and are testing it in a field trial.

(mi)

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Addresses

Project management
Universität Siegen, IFT

Project partner
Senvion Deutschland GmbH

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