By using longer blades and fine-tuning them into more aerodynamic shapes, manufacturers are squeezing more and more performance out of their wind turbines. But the background noise caused by stalling continues to grate on some people’s nerves. The phenomenon requires more than just a psychologist.
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 relation to their capacity. A Vestas V 90 reaches its maximum noise emission at 7.5 m/s, an E 82 at 9 m/s”, explains Bunk.
For noise assessors, this is the decisive factor. These noise emission values are guaranteed by the manufacturers. When the noise assessors predict the noise emissions of a wind farm or come to take measurements after its completion, they measure how many times that level is reached per year, and how much of that noise could reach people living nearby. However, in many European countries the measurements are only conducted at a standard height of ten metres, not at hub height.
The research continues
For the AM phenomenon on the other hand, there is not yet a standard process to measure the noise, which makes comparisons difficult. “In the UK, AM was a big issue last year, which led to some measurements being taken, including under laboratory conditions, to determine the cause of the anomaly. Experts believe that it is probably caused by stalling, because the angle of attack of the blades is hard into the wind”, says Sylvia Broneske of the acoustics consultancy Hayes McKenzie Partnership Ltd.
In response to protests, the industry association RenewableUK commissioned Temple Group to conduct a study (Turbine Amplitude Modulation: Research to Improve Understanding as to its Cause and Effect) that made use of data from the Danish institute Risø DTU. Temple Group came to the conclusion that AM should be differentiated into normal and abnormal sounds. The normal swishing sound is caused by air friction, turbulence or the airflow being blocked by the tower. The abnormal noises on the other hand could be caused by an enormous number of possible combinations. These could result from the location, the height of the hub or the atmospheric stratification of the wind.
In the vicinity of the turbine, the noise reaches at most 5 dBA and is hardly an irritation, according to the report. This conclusion is based on a study by the University of Salford from 2007 that looked at 113 wind farms and only detected AM at four of them. “For us, AM is no longer a big issue, even though public interest remains high. Some people seem to confuse noise pollution and normal noise. This has to be looked into project by project”, says Robert Norris of RenewableUK.
Until recently, local authorities adhered to guidelines from 1997. The ageing document was replaced in May 2013 by the IOA Good Practice Guide developed by the Institute of Acoustics and a working group. According to these new guidelines, wind turbines can emit 35 to 40 dBA during the day and up to 43 dBA at night, and may exceed these figures by up to 5 dBA depending on the ambient noise level. Despite numerous objections, the new guidelines do not include a minimum distance to residential buildings or standardised methods for measurement.
A case for psychologists
The idea that people might be confusing general noise with noise pollution also emerges from a new study by the German Foundation for the Environment (Deutsche Bundesstiftung Umwelt). The study conducted interviews with local residents and took measurements. The swishing could be heard at nine Enercon turbines in the wind farm at Wilstedt in Lower Saxony.
At a maximum of 33 dBA, the wind farm is well within legal noise levels, and yet the AM phenomenon is causing annoyance. Over the course of two years, 212 local residents were therefore interviewed, and their responses compared with those from residents near another 13 wind farms. In the first round, 30 % of those interviewed were not at all bothered by the wind farm, 25 % felt somewhat bothered and 10 % were very disturbed. The latter had trouble sleeping and struggled with negative moods.
After the first round of interviews, the turbines were run in various modes over a period of six months and a selection of residents regularly questioned. The results were baffling: “Changing the operational parameters had absolutely no effect. There was neither a linear nor systematic development. We had expected a positive effect. Also, the distance to the turbines made no difference whatsoever. The same was found in samples from other sites”, explains Project Manager Johannes Pohl, a psychologist at the University of Halle-Wittenberg.
A matter of habituation
The results of the second round of interviews were just as puzzling. This time the researchers included the nuisance from traffic. Only 7 % of respondents still felt very disturbed by the wind farm, but 16 % were very disturbed by the traffic in Wilstedt. According to the psychologist, “it seems that most people can deal with the noises better over time. So the operation of the wind farm has a positive effect in the longer term.” He offers an explanation: “The higher values at the start of the study could be related to the fact that the planning and construction phase had led to an extended period of stress.”
Despite this, he warns against ignoring the health effects on the minority who still feel bothered by the noise. This view is shared by Joachim Gabriel, noise assessor at the German Wind Energy Institute, who was in charge of the measurements for the study at the wind farm and at the inhabitants’ houses. “Wilstedt is not an isolated case. The same phenomenon appears at other wind farms. That is why it is important to develop a standardised method for measuring amplitude modulation. This would give us a tool for long-term measurements”.
The manufacturers are also working on identifying the causes of the problem. “We take the complaints very seriously and are working to get to the bottom of the phenomenon”, explains Felix Rehwald, press spokesperson for Enercon. He believes that in addition to long-term studies, continual noise measurements at the turbines could be helpful. There would be more objective data to go by instead of subjective assessments.