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Electromagnetic interference (EMI) is any type of interference that can potentially disrupt, degrade or interfere with the effective performance of an electronic device. Modern society is dependent on the use of devices that utilise electromagnetic energy such as power and communication networks, electrified railways, and computer networks. During the generation, transmission and utilisation of electromagnetic energy, the devices generate electromagnetic disturbance that can interfere with the normal operation of other systems.
Wind turbines can potentially disrupt electromagnetic signals used in telecommunications, navigation and radar services. The degree and nature of the interference will depend on:
Interference can be produced by three elements of a wind turbine: Tower, rotating blades and generator. Tower and blades may obstruct, reflect or refract the electromagnetic waves. Modern blades are typically made of synthetic materials which have a minimal impact on the transmission of electromagnetic radiation. The electrical system is not usually a potential problem on telecommunications because interference can be eliminated with proper nacelle insulation and good maintenance.
Interferences to mobile radio services are usually negligible. Interferences to TV signals have been clearly minimised with the substitution of metal blades with synthetic materials. However, when turbines are installed very close to dwellings, interference has been proven difficult to rule out.
The interference area may be calculated using the Fresnel zone. This area is around and between the transmitter and receiver depending on transmission frequency, distance between them and local atmospheric conditions.
Technical mitigation measures on TV interference can be applied during the planning stage, siting the turbine away from line-of-sight of the broadcaster transmitter. Once the wind farm is in operation there are also a set of measures to mitigate the interference.
There is a common agreement that adequate design and location can prevent or correct any possible interference problems at relatively low cost using simple technical measures, such as the installation of additional transmitter masts. Interference on communication systems is considered to be negligible because it can be avoided by careful wind farm design.
There is a rough consensus about which are the most important environmental threats, and what are their general influences on biological diversity. The continuous deterioration of natural habitats and the increasing number of wild species which are seriously threatened has prompted governments to protect the environment.
There are many types of protected areas at national and regional levels across the countries. At European Community level, the Birds Directive (1979) and the Habitats Directive (1992) are the base of Europe's nature conservation policy.
The Birds Directive is one of the most important tools to protect all wild bird species naturally living or migrating through the European Union. The directive recognises that habitat loss and degradation are the most serious threats to the conservation of wild birds. The Birds Directive has identified 194 species and sub-species (listed in Annex I) as particularly threatened and in need of special conservation measurements.
The aim of the Habitats Directive is to promote the maintenance of biodiversity by preserving natural habitats and wild species. The Annex I includes a list of 189 habitats and the Annex II listed 788 species to be protected by means of a network of high values sites. Each Member State has to define a national list of sites for evaluation in order to form a European network of Sites of Community Importance (SCIs). Once adopted, SCIs are designated by Member States as Special Areas of Conservation (SACs), and, along with Special Protection Areas (SPAs) classified under the EC Birds Directive, form a network of protected areas known as Natura 2000.
The development of wind farms in natural reserves areas should be assessed on site-specific and species-specific criteria to determine whether the adverse impacts are compatible with the values for which the area was designated.
Of special importance is the mandatory requirement of the Habitats Directive to include indicative "sensitivity" maps of bird populations, their habitats, flyways and migration bottlenecks as well as an assessment of the plan/programme's probable effects on these in the SEAs and AAs procedures. These maps should provide enough information about feeding, breeding, moulting, resting, non-breeding and migration routes to guarantee biodiversity conservation.
Offshore wind energy is a renewable technology capable of supplying significant energy in a sustainable way. According to EWEA estimates, between 20 GW and 40 GW of offshore wind energy capacity will be operating in the European Union by 2020. This capacity could meet more than 4% of EU electricity consumption in 2020. The total offshore installed capacity in Europe at the end of 2007 was almost 1,100 MW distributed in the coastal waters of Denmark, Ireland, Netherlands, Sweden and United Kingdom, representing almost 2% of the total wind energy (56,536 MW) in the European Union.
Offshore wind projects are more complex than onshore ones. Offshore developments include platforms, turbines, cables, substation, grid, interconnection and shipping, dredging and associated construction activity. The operation and maintenance activities include the transport of employees by ship and helicopter and occasional hardware retrofits.
From an ecological point of view, shallow waters are usually places with high ecological value and are important habitats for breeding, resting and migratory seabirds. Close participation and good communication between the countries involved in the new developments is essential to reduce environmental impacts from several wind farms in the same area.
Most of the experience gained in offshore wind energy comes from several years of monitoring three wind farms in Denmark (Middelgrunden, Horns Rev and Nysted) installed between 2001 to 2003. Valuable analysis has also been carried out by the Federal Environment Ministry (BMU) of Germany through technical, environmental and nature conservation research about offshore wind energy foundations.
Offshore wind farms usually have more and bigger turbines than onshore developments. However, visual impact is lower due to the greater distance from the coastline. Nevertheless, the coastal landscape is often unique and provides some of the most valued landscape, thus wind developments can be sensitive.
Visual impact of offshore wind farms can affect three components of the seascape:
Figure 2.1 Components of seascape.
Offshore wind farms involve several elements which have influence on the character of the produced visual impact:
Just like in onshore developments, ZTV zones, photomontages and video-montages tools are used to predict the potential effects of new offshore wind developments.
The visibility assessment on offshore developments includes the extent of visibility over the main marine, coastline and land activities (recreational activities, coastal populations and main road, rail and footpath). The effects of the curvature of the earth and lighting conditions are relevant in visibility of offshore wind farms. Rainy and cloudy days result in less visibility. Experience to date on Horns Rev proves that a wind farm is much less visible than the 'worst case' clear photomontage assessment, due to prevailing weather conditions and distance.
The magnitude of change in the seascape with the construction of a new offshore wind farm is dependent of several parameters such as distance, number of turbines and the proportion of turbine that is visible, weather conditions and the navigational lighting of turbines. The distance between observer and wind farm usually forms the strongest influence on the visual impact perception. Nevertheless, changes in lighting and weather conditions vary considerably the visual effects at the same distance.
The indicative thresholds established for high sensitive seascapes during the DTI study on three SEA areas in U.K. are shown in table 2.3:
Table 2.3: Thresholds for seascapes
|<13km possible major visual effects|
|13-24km possible moderate visual effects|
|>24km possible minor visual effects.|
More recently research on visual assessment by Bishop and Miller, 2005, found that distance and contrast are very good predictors of perceived impact. The study about North Hoyle wind farm at 7 km off the coast of Wales showed that in all atmospheres and lighting conditions (except a stormy sky), visual impacts decreased with distance. However, visual impact increased with increasing contrast. Further research is needed to analyse the dependence of visual effects on turbine numbers, orientation and distribution.
When several developments concur in the same area, cumulative effects may occur by additional impacts of new wind farms. The degree of cumulative impact is a product of the number of wind farms and distance between them, the siting and design of the offshore wind farms, the inter-relationship between their ZTVs, and the overall character of the seascape and its sensitivity to wind farms.
The Danish Energy Agency (DEA) has reported an absence of negative press during the development of Nysted and Horns Rev offshore wind farms. Opinion polls showed better acceptance levels for the projects in the post-construction phase.
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