Forthbank Wind Energy Development - Partnerships for Renewables

Forthbank Wind Energy Development
14 Shadow Flicker
14.1 Introduction
14.1.1 Shadow flicker occurs under specific conditions when the sun passes behind the rotating
blades of a wind turbine and casts a moving shadow over a nearby building. When this
shadow is viewed through a narrow opening, such as window, the phenomenon is known as
shadow flicker.
14.1.2 Shadow flicker within properties occurs if a wind turbine is in close proximity to and of a
particular orientation with a nearby building. Generally it will not occur where there are trees or
mature vegetation or other obstructions including buildings and structures between the turbines
and the building receptor; if windows facing a turbine are fitted with blinds or curtains; or if the
sun is not shining brightly enough to cause shadows from a turbine.
14.1.3 The shadows cast by a wind turbine will vary in length according to the sun’s altitude and
position according to its bearing. The likelihood of shadow flicker occurring and the duration of
such an effect depend upon the following:
• The orientation of the building relative to the turbine: in the UK, only properties
within 130 degrees either side of north, relative to a turbine, can be affected, as
turbines do not cast long shadows on their southern side;
• Distance from turbine(s): the further the observer is from the turbine, the less
pronounced the effect would be. At long distances, turbine blades do not completely
cover the sun, but only partially mask it, resulting in a significantly weaker shadow.
PAN 45 states that shadow flicker generally only occurs at nearby dwellings within a
zone of ten rotor diameters from a turbine.
• Wind direction: the turbine will not always be directed towards the receptor i.e.
‘square-on’ which is the worst case scenario, therefore the effect will be minimised;
• Turbine hub-height and rotor diameter;
• The size of habitable room windows facing the turbine;
• Time of year;
• The proportion of day-light hours in which the turbine operates;
• The frequency of bright sunshine and cloudless skies (i.e. cloudy days reduce the
likelihood of effects occurring); and
• The prevailing wind direction.
November 2010 Chapter 14 Page 1
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