Wind Power in the UK (PDF). - Sustainable Development Commission

sd.commission.org.uk

Wind Power in the UK (PDF). - Sustainable Development Commission

Annex C: Aviation and communicationtransmission links is correspondingly splitbetween two companies – Crown CastleInternational (CCI) and NTL.Because wind farms are usually situated inrelatively sparsely populated areas, the numbersof people affected are usually small. There isnow extensive experience in the industry ofwind farm developers entering into planningagreements to fund studies of TV receptionquality and any mitigation required. This cantake the form of installation of more sensitivereceiver antennae for individual subscribers;moving antennae to receive from a differentsource transmitter; or installing a localcommunity re-broadcast facility.The fixed microwave and UHF transmissionnetwork can present more widespread issues forwind energy developments. These travel instraight lines between two fixedtransmitter/receiver points. Vulnerability tomulti-path effects is determined by thefrequency of the signal and the length of thelink path. The lower the frequency and thelonger the link, the more risk there is of multipatheffects. Consequently, long-distance links atUHF frequencies require much larger clearancezones than short microwave links.In 2002 the Radiocommunications Agency – nowpart of Ofcom – produced a methodology forcalculating the minimum separation distancebetween a radio link and a wind turbine, usingas its basis the concept of the Fresnel Zone 2 . Asan example, the Ofcom formula calculates that a20km long microwave link at 7 GHz wouldrequire a clearance of 21 metres between thecentre of the link path and any part of a windturbine at the midpoint of the link, but a smallerclearance towards either end. At 2 GHz,however, the same link would require 39 metresclearance.As can be seen from the example above, theOfcom recommended clearances are relativelysmall, and would permit constructing a windfarm directly in the path of a microwave link ifthe turbines were placed to avoid the calculatedFresnel Zone.The consultation process for potential wind farmimpacts on television is well-established butbecause of the split responsibilities between theBBC, two parts of Ofcom, Crown Castle and NTL,it lacks integration. Ofcom licenses most fixedradio links in the UK and this covers some of thetelevision transmission network. However theyare not responsible for UHF RBLs. It is thereforenecessary to consult all four television bodiesplus the fixed link department of Ofcom toobtain an assessment of the likely impact of awind farm proposal on television. There is alsouncertainty derived from differences in policy onthe response to consultations. Some of thetelevision consultee organisations will providepreliminary pre-planning assessments whichenable developers to assess project risk at anearly stage. Others provide no response untilafter a planning application is submitted.Ofcom recommendations not alwaysfollowedThe problem, however, is that thetelecommunications industry, includingtelevision broadcasters, have not generallytaken up the Ofcom recommendations. This isbecause of a number of uncertainties about theprecise effects of wind turbines on radio links,notably:• The potential for more complex multi-patheffects to occur in multiple-turbine wind farms• A belief that the Ofcom formula is atheoretical minimum and that engineeringpractice ought to build in a 'buffer' to takeaccount of uncertainties144 Wind Power in the UK sustainable development commission

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