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Noise from wind developments has been one of the most studied environmental impacts of this technology. Noise, compared to landscape and visual impacts, can be measured and predicted rather easily.
Wind turbines produce two types of noise: mechanical noise from gearboxes and generators and aerodynamic noise from blades. Modern wind turbines have virtually eliminated the mechanical noise through good insulation materials in the nacelle. Aerodynamic noise is the highest contributor to the sound. The aerodynamic noise is produced by the rotation of the blades generating a broad-band swishing sound and it is a function of tip speed. Design of modern wind turbines has been optimised to reduce aerodynamic noise. This reduction can be obtained in two ways. Decreasing rotational speeds under 65 m/s at the tip and using pitch control on upwind turbines which permits the rotation of the blades along their long axis.
At any given location the noise within or around a wind farm can vary considerably depending on a number of factors including the layout of the wind farm, the particular model of turbines installed, the topography or shape of the land, the speed and direction of the wind and the background noise. The factors with the most influence on noise propagation are the distance between the observer and the source and the type of noise source.
The sound emissions of a wind turbine increase as the wind speed increases. The background noise will typically increase faster than the sound of the wind turbine, tending to mask the wind turbine noise in higher winds. The sound emissions decrease when the distance from wind turbines is increased.
Noise levels can be measured and predicted but public attitude towards noise depends heavily on perception. Sound emissions can be accurately measured using standardised acoustic equipment and methodologies (ISO Standards, IEC Standards, ETSU, etc). Levels of sound are most commonly expressed in decibels (dB). The predictions of sound levels in future wind farms are of the utmost importance in order to foresee the noise impact.
The next Scottish Government table compares noise generated by wind turbines with other everyday activities.
Table 2.1: Comparative noise for common activities, CIEMAT :
level dB (A)
|Threshold of hearing||0|
|Rural night-time background||20-40|
|Wind farm at 350m||35-45|
|Busy road at 5km||35-45|
|Car at 65 km/h at 100m||55|
|Busy general office||60|
|Truck at 50km/h at 100m||65|
|Pneumatic drill at 7m||95|
|Jet aircraft at 250m||105|
|Threshold of pain||140|
When there are people living near a wind farm, care must be taken to ensure that sound from wind turbines should be at a reasonable level in relation to the ambient sound level in the area. Rural areas are quitter than cities, so the background noise is usually lower. However, there are also noisy activities in the countryside such as agricultural, commercial, industrial and transportation. On the other hand, wind farms are located in windy areas, where background noise is higher, thus this background noise tends to mask the noise produced by wind turbines. The final objective is to avoid annoyance or interference in the quality of life of the nearby residents.
Noise levels can be measured and predicted but public attitude towards noise depends heavily on perception. Due to the wide variation in the levels of individual tolerance for noise, there is no completely satisfactory way to measure the subjective effects of noise, or the corresponding reactions of annoyance and dissatisfaction. The individual annoyance for noise is a very complex topic. Dose-response relationship studies have demonstrated a correlation between noise annoyance with visual interference and the presence of intrusive sound characteristics. In the same way the annoyance is higher in a rural area than in a suburban area and higher also in complex terrain in comparison with flat ground.
Low frequency noise (LFN), also known as infrasound, is used to describe sound energy in the region below about 200 Hz. LFN may cause distress and annoyance to sensitive people. For this reason, LFN has been widely analysed. The most important finding is that modern wind turbines with the rotor placed upwind produce very low levels of infrasound typically below the threshold of perception. A critical survey of all known published measurement results of infrasound from wind turbines concludes that from upwind turbines infrasound can be neglected in evaluating the environmental effects of wind turbines.
Experience acquired while developing wind farms suggests that noise from wind turbines is generally very low. The comparison between the number of noise complaints about wind farms and other types of noises indicates that wind farm noise is a small scale problem in absolute terms. Information from the United States also suggests that complaints about noise from wind projects are rare, and can usually be satisfactorily resolved.
National authorities consider the development of wind farms in their planning policies for wind energy projects. Decisions on siting should be made with consideration to other land users.
The administrative procedures needed to approve wind plants in each specific place have to be taken into account from the beginning of the project planning process. Regional and local land use planners must decide whether a permit is compatible with existing and planned adjacent uses, whether it will modify negatively the overall character of the surrounding area, disrupt established communities, or if it will be integrated into the existing landscape. Developers, in the very early planning stage, should contact the most relevant authorities and stakeholders in the area: Ministry of Defense, civil aviation authorities, radar and radio communication suppliers, the grid company, environmental protection authorities, the local population and relevant non governmental associations, among others.
The authorities involved in reviewing and making land use decision on projects must coordinate and communicate with each other throughout the project. At the same time, local citizen participation as well as good communication with the main stakeholders in the area would help to obtain a successful wind development.
Special attention must be paid on nature reserves, their surrounding zones and habitats of high value for nature conservation. There are additional obligations for assessment when Ramsar sites or Natura 2000 sites could be significantly affected by wind energy developments. The project or plan will only be approved if there is not an adverse effect on the integrity of the site. If it cannot be established that will be no adverse effects, the project may only be carried out if there are no alternative solutions and if there are imperative reasons of public interest.
Recently, a new concern has been raised: wind farm installations over peatlands. Peatlands are natural carbon storage systems with a delicate equilibrium of waterlogging. According to UNEP-GEF, peatlands cover only 3% of the world's surface, but store the equivalent of 30% of all global soil carbon, 75% of all atmospheric carbon. The impacts associated with drainage are: carbon dioxide and methane emissions, erosion and mass movements and dissolved organic carbon. The consequence is the loss of its capability of acting as a carbon sink. Besides, the EU Habitats Directive has designated several grasslands formations as special areas of conservation. In those areas, the Member States have the responsibility to apply the necessary conservation measures for the maintenance or restoration of the natural habitats and/or the populations of the species for which the site is designated. The Scottish Government has recently developed an approach to calculate the impact of wind energy on organic soils. This method permits the calculation of potential carbon losses and savings of wind farms, taking into account peat removal, drainage, habitat improvement and site restoration. The method proposes to integrate the carbon losses by peatland use in the overall Life Cycle Assessment (LCA) of the wind farms, computing the global carbon saving by the use of wind energy and subtracting the carbon losses associated with wind farm installations. The study also provides some recommendations for improving carbon savings of wind farm developments:
Another issue is the interaction between tourism and wind energy developments. Many of the tourist areas are located in beautiful and/or peaceful landscapes. Wind power plants on the landscape could reduce the attractiveness of the natural scenery. The most recent study, carried out by Scottish Government has analysed the impacts of wind farms on the tourism industry as well as a review of 40 studies from Europe, US and Australia. The conclusions from the review can be summarised in this way:
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