Shadow flicker happens when the sun is low in the sky (usually when rising or setting) and shines on a building from behind a turbine. This can cause the shadow of the rotating turbine blades to be cast onto the building and through windows.  Many residents who suffer from this phenomenon describe it as ‘like a light being flicked on and off over and over again’.

Wind farm developers use computer software programs to predict the number of hours, the time of year and number of weeks per year that a property could be affected.  Worst case scenarios are usually prepared, but in reality the actual time someone may be affected depends on several factors including weather conditions i.e. there may be cloud cover so the flicker effect is not noticed.

According to PAN 45 (a Scottish Government guideline):-

• Shadow flicker only occurs inside buildings where the flicker appears through a narrow window opening;
• A general rule of ten rotor diameters should be used for separation distance from a turbine position to a dwelling.

The document goes on to say “In most cases however, where separation is provided between wind turbines and nearby dwellings (as a general rule 10 rotor diameters), “shadow flicker” should not be a problem.” (Paragraph 64)”

For turbines of an overall height of 125 – 130m, with a rotor blade diameter of 100m, this would equate to a 1km setback.  Shadow flicker in the UK only affects properties which lie within 130° of north.  Anyone living to the SSE/S/SSW would not be affected.

A recent report for the DECC (Department of Energy and Climate Change), Update of UK Shadow Flicker  Evidence Base 2011 by Parsons Brinckerhoff, quotes research from the 1990’s which all recommend a distance of 10 rotor diameters between turbine and properties to reduce shadow flicker effect.

A Case of Shadow Flicker / Flashing: Assessment and Solution, Clarke A.D. (1991), asserts that:

“The minimum separation distance for wind turbines from habitations should be approximately 10 blade diameters. This is emerging from experience and research as a standard guideline, in order to reduce problems of visual impact, noise, shadow disturbance, and safety”.

Planning for Wind Energy in Dyfed, Taylor D. & Rand M. (1991)

“Wind turbines can cause shadow disturbance over a large area around a turbine, but the duration is likely to be limited. From the data presented above it is possible to deduce that the shadow effect can be reduced to relatively short periods of the year (30 minutes a day for 10-14 weeks a year) when spacings of 10 rotor diameters to the nearest habitation are employed.”

“The siting of wind turbines less than 10 rotor diameters from habitations should be discouraged due to the increased duration of shadow effects.”

The DECC report can be found here

Mitigation measures offered by wind farm companies include installing blinds in the affected property, switching the lights on to lessen the effect, planting trees and bushes in front of the window to reduce the effect or turning the turbines off.

Shadow flicker cannot bring on epileptic fits as has been previously claimed but can trigger migraines and headaches for people who are sensitive.   The DECC report explores research carried out in 1999 and 2000 about the effects of shadow flicker on people, which found over time it may “meet the criteria of a significant nuisance.”

Below are some of the findings:-

• The extent of the impact that shadow flicker causes is given in a psychology study (Pohl, 1999). This study concludes that the shadow flicker effect did not constitute a significant harassment. However, under specific conditions the increased demands on mental and physical energy, indicated that cumulative long-term effects might meet the criteria of a significant nuisance. This demonstrates the need to reduce the impact where possible.

• The laboratory study by the University of Kiel (Pohl et al 2000) noted that even a one-off exposure to 60 minute duration of shadows can cause stress reactions. For precaution, shading duration is therefore limited to 30 minutes per day.

• Scientific research [no reference given in text] has demonstrated experience that optical emissions in the form of periodic shadows can result in considerable harassment effects.

All the evidence indicates that turbines should not be erected within 10 rotor diameters of residential properties but this is being flouted by wind farm companies who are regularly applying for planning permission to build just 400m -500m from dwellings. 

The profits to be had from wind power are now so great that companies will turn off the turbines for limited periods if it means they are less restricted on where they can build. 

You may think that reasonable but shadow flicker, when added to all the other cumulative effects, such as noise, loss of amenity, and lower property values all add up to an unacceptable situation, currently sanctioned by the planning system and perpetuated by uncaring developers.
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