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Tuesday, 11 March 2014
16:30 - 18:00 Wind turbine noise: How to avoid disturbing the neighbours
Resource Assessment  


Room: Ponent
Session description

With a new version of IEC 61400-11 published this year, it may be thought that wind turbine noise emission is well understood. However, wind farm developers still receive complaints from residents about 'amplitude modulation' (AM) noise, a subject not covered by the IEC standard. Amplitude modulation will therefore be one of the main topics in this session. This session will also explore different sound propagation models and the impact of environmental conditions on propagation. Another topic for this session is how to optimise wind turbines and wind farms for sites with noise restrictions.

Learning objectives

  • Understand why it is important to consider noise for resource assessment
  • Understand how a wind farm as a whole can be optimised to maximise performance with noise constraints
  • Understand what amplitude modulation is and why it is a concern for residents near wind farms
  • Understand the role that propagation modelling plays in the prediction of far field noise
Lead Session Chair:
Saskia Honhoff, GE Energy, Germany
Lukas Mylonas University of Uppsala, Germany
Co-authors:
Lukas Mylonas (1) F P Bahri Uzunoglu (1)
(1) University of Uppsala, Mittenaar(Ballersbach, Germany

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

Mr. Mylonas has gained extensive knowledge on the wind energy sector through his studies and work experience. He recently completed a six month internship at juwi GmbH, where he worked in the wind energy site assessment department. He has successfully defended three scientific papers related to renewable energy topics. Besides a Master in Wind Power Project Management, he holds a Master in Environment in Resource Management, a post-graduate degree in Energy and Climate issues and a Diploma in Economic Sciences. His research at the moment is focused on noise prediction models for wind turbines.

Abstract

Assessment of noise prediction models for long-range sound propagation of wind turbines over water

Introduction

Wind turbine noise is a re-emerging issue in the wind industry. As competition for good wind sites on land is rising, offshore projects in coastal areas seem as a reasonable alternative. Sound propagation over water and other hard surfaces is especially relevant for wind turbine noise because sound can travel over longer distances. Moreover various meteorological factors and phenomena are interfering with the propagation of sound. The prediction tools commonly used by developers are only partially taking these parameters into account. Therefore more accurate prediction tools are needed that can account for the interactions of different meteorological parameters.

Approach

First, different models for calculating wind turbine noise over water were assessed. These models are the ISO 9613-2 standard for outdoor noise prediction, the Danish method and the Swedish method for wind turbines noise estimation over water.

Then a Parabolic Equation scheme is used to examine different meteorological conditions and phenomena on a flat reflecting surface like the sea. In particular the experiments include meteorological conditions with different levels of sound refraction in a non-turbulent and turbulent atmosphere. In addition a meteorological phenomenon called the low-level jet is investigated.



Main body of abstract

Parabolic equations approximating the wave equation are considered accurate prediction tools for outdoor sound propagation. The advantage of this method is that it can incorporate changing horizontal and vertical sound speed profiles, turbulences, varying surface impedance and roughness.

In the experiments conducted for this study using a matlab code, different sound speed profiles were employed simulating atmospheres with different levels of refraction. Refraction is a phenomenon which is due to wind speed and temperature gradients. These gradients cause enhancement of sound levels by bending sound waves towards the ground. Moreover hard surfaces such as water are reflecting sound waves and can thus create multiple reflections, leading to an increased propagation path of sound waves. It is therefore important to have prediction tools that integrate these effects in their calculations.

Furthermore turbulences are especially relevant for sound propagation as they will cause diffraction of sound waves (attenuation of sound). For that purpose the Monin-Obhukov similarity theory was computed into the sound speed profile of the parabolic equation and tested for different cases. Special focus was given to the low-level jet phenomenon.

Low-level jets are characterised by high wind speeds at low altitudes and therefore create conditions with strong refraction. In order to evaluate the interaction of strong refracting atmospheres and turbulences, a non-turbulent low-level jet profile and a turbulent low-level jet profile were compared. The results showed that turbulences can significantly reduce sound pressure levels under these conditions.





Conclusion

Parameters such as wind speed and temperature gradients, turbulences and surface impedance have a significant impact on sound propagation. As the experiments in this study reinforced it is important to take these parameters into account when estimating wind turbine noise. The assessment of prediction models, suggested by authorities today, highlighted the weaknesses of these methods to incorporate the above parameters.

Nonetheless, relatively more complex models such as the parabolic equation require experienced users and a large computational time. Further effort is required to enable prediction tools with a higher level of accuracy to be established in the wind energy sector.




Learning objectives
The outcome of this study emphasizes the importance of taking into account meteorological conditions when calculating wind turbine noise. Noise prediction tools used by developers need to be able to consider these effects in order to acquire reliable results. For this purpose, the parabolic equation is compared to standards incorporating turbulence that significantly reduce sound pressure levels and low level jets.