What is shadow flicker?

Shadow flicker occurs under a special set of conditions when the sun passes behind the hub of a wind turbine and casts a shadow over neighbouring properties. When the blades rotatate, shadows pass over the same point causing an effect called 'shadow flicker'. Shadow flicker effects occur in various situations: travelling by road through a tunnel or under overhanging trees (dappled shadow effects), or standing within the shadowed area of wind turbine blades.

Shadow flicker within houses occurs if a wind turbine is close enough to and of a specific orientation with, a nearby house. It will not happen where there is vegetation or other obstructions between the turbines and the house; if windows facing a turbine are fitted with blinds or shutters; or if the sun is not shining brightly enough to cause shadows from a turbine.

The seasonal timing and duration of this effect can be accurately calculated from the geometry of the machine, its orientation relative to nearby houses and the latitude of the potential site, using computer software. Any properties which may potentially be affected can be identified and the risk calculated. By using this software Ecotricity chooses the sites and layouts for wind parks which minimise the risk of shadow flicker occuring at residential properties. In the unlikely event that shadow flicker does significantly affect residential properties, we will undertake measures to rectify the problem to the full satisfaction of the people involved. This is likely to involve pre-programming the turbine with dates and times when shadow flicker would cause a nuisance. A photos sensor cell would be used to monitor sunlight. The turbine would then be shut down, when the strength of the sun, wind speed and the angle and position of the sun combines to cause a flicker nuisance.

The maximum potential flicker frequency from the E66 turbine is 1.1 flickers per second. This is outside of the frequency which could cause health problems, established by the Health and Safety Executive at 2.5-40 flickers per second (http://www.hse.gov.uk/lau/lacs/51-1.htm).