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The wind power industry has grown accustomed to conflicts with environmentalists and groups of local residents. Noise pollution in particular has long been a hot issue. Whenever this topic arises, the debate quickly moves into the broad field of psychology. Noise pollution always seems to have a subjective component, and there are very few really empirical studies regarding the possible health effects on local residents. The ongoing debates about airborne noise or the possible effects of low-frequency noise have continually led to calls for the distance between wind farms and buildings to be increased. Finland is a case in point: Jari Keinänen, Director of the Ministry of Social Affairs and Health, has just declared noise to be the single biggest danger to people residing near wind farms. In June, he called for the current minimum distance of 500 m to be increased to 2,000 m. “It may be possible to go closer, but only when there are reliable figures for an impact assessment”, he said. There is a similar trend in France, where doctors are calling for a minimum distance of 1,500 m. A decree from 2011 already introduced stricter rules. In areas where noise emissions are regulated, wind farms may not exceed 35 dBA. To this value 3 to 5 dBA are added depending on the prevailing noise in the surrounding area. The total may only be exceeded at fixed times of day. There is also a maximum noise level of between 60 and 70 dBA. These values are measured in a fixed area around each turbine, the radius of which is calculated from the hub height and half the length of the rotor blades. The wind farm must remain within the noise limit at all points within this zone. A new problem Manufacturers like Enercon have started providing specially programmed control systems for such difficult markets. But now the industry is having to deal with a new theme called amplitude modulation (AM) – an abbreviation better known from the frequency settings on radios. The phenomenon appears as a regular “swish” sound at the turbine blades. At least, that is how some local residents describe the noise, which sometimes prevents them from sleeping. This is the result of tiny changes each second to an otherwise constant sound level. Experts suspect that the noise is related to the new, aerodynamic designs of rotor blades and their angle to the wind. “It has to do with stalling effects at the blades’ surfaces. The blades of a large wind turbine cut through a high wind profile containing large differences. A lot more research still needs to be done”, says Oliver Bunk, Head of the wind turbine section at Kötter Consulting Engineers. The phenomenon is especially vexing because it occurs within the legal noise limits but still leads to problems with residents. “Wind turbines have actually become much quieter in … and all were found to be within legal limits. Wind Edition 43


SWE 2014 08
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