There are two different sources of noise in a wind turbine: aerodynamic, originating from the motion of air around the blades, and mechanical, caused by the motion of the mechanical and electrical components.
Dominant in large wind turbines, aerodynamic noise sources are harder to identify and control.
Andreas Fischer, a researcher from the Technical University of Denmark (DTU), the largest research centre for wind energy worldwide, explains: “Aerodynamic noise has two main mechanisms: interaction of the blade’s leading edge with atmospheric turbulence, called inflow noise, and the interaction of the sharp trailing edge with the turbulence created on the surface of the blade, called trailing edge noise”.
“The inflow noise is strong in the low frequencies, whereas the trailing edge noise is stronger in the higher frequencies where the human ear is most sensitive. We therefore need to reduce the trailing edge noise in order to make wind turbines quieter for human perception”, he adds.
Measuring the noise of wind turbines is an important field of research. “It can be reduced by adequately pitching the blades and lowering the rotational speed of the drive train. However this can lead to a drop in power generation,” explains Juan Carlos García Andújar, technical unit manager from Gamesa, one of the world's leading manufacturers of wind turbines.
The Spanish company is developing devices to tackle the noise of wind turbines under the European project Windtrust, which aims to reduce the cost of wind energy generation by further improving the reliability of the turbines.
“To mitigate the trailing edge noise, serration devices have been added to the outer part of the blade. They reduce sound radiation by modifying how the pressure waves along the airfoils act with the trailing edge”, says García Andújar, “Tests on an onshore 2MW prototype turbine have shown that a reduction of around 2 dB can be achieved by using serrations, but each type of blade requires specific serration geometries. For the future, new geometries will be defined and tested for other blades”.
DTU is also part of the Windtrust research consortium. “Modelling the effects of the serrated trailing edge in reducing noise remains challenging, but we have been working on this and have made some progress”, says Fischer.
“We also want to progress with predicting the noise reducing capabilities of other devices such as trailing edge brushes. Inflow noise modelling will get some attention in the future. Another option is to develop a more efficient control strategy for wind power with the aim of reducing noise without affecting power generation. At the moment the loss in generation is significant when the turbine is in a low-noise mode”, he adds.
It is important to enhance the technical aspects of wind turbines as wind power is a fast growing field. “I hope public annoyance will be overcome so that wind energy can be developed without hindrance. Wind energy is a clean form of energy. In the future, we need a mix of sustainable energy sources to avoid negative effects on the environment,” concludes Fischer.
By Jessica Haapkyla
7 July 2016