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Wind NI | Shadow Flicker from Wind Turbines

Advice Section

Shadow Flicker from Wind Turbines

Posted on: February 4th, 2009

Wind turbines, like other tall structures will cast a shadow on the neighbouring area when the sun is visible. If you live very close to the wind turbine, it may be annoying if the rotor blades chop the sunlight, causing a flickering (blinking) effect while the rotor is in motion. A bit of careful planning, and the use of good software to plan your wind turbine site can help you resolve this problem, however. If you know where the potential flicker effect is of a certain size, you may be able to place the turbines to avoid any major inconvenience for the neighbours.

Predicting Shadow Flicker

Fortunately, we are able to predict quite accurately the probability of when and for how long there may be a flicker effect. We may not know in advance whether there is wind, or what the wind direction is, but using astronomy and trigonometry we can compute either a likely, or a “worst case” scenario, i.e. a situation where there is always sunshine, when the wind is blowing all the time, and when the wind and the turbine rotor keep tracking the sun by yawing the turbine exactly as the sun moves.

Figuring out the exact shape, place, and time of the shadow from a wind turbine requires a lot of computation, but can be done using a professional wind software programme and can do this very accurately, even in hilly terrain, and with house windows of any size, shape, location and inclination facing in any direction.

Taken from PPS18

A104. Under certain combinations of geographical position and time of day, the sun may pass behind the rotors of a wind turbine and cast a shadow over neighbouring properties. When the blades rotate, the shadow flicks on and off; the effect is known as ‘shadow flicker’. It only occurs inside buildings where the flicker appears through a narrow window opening. A single window in a single building is likely to be affected for a few minutes at certain times of the day during short periods of the year. The likelihood of this occurring and the duration of such an effect depends upon:

  • the direction of the residence relative to the turbine(s);
  • the distance from the turbine(s);
  • the turbine hub-height and rotor diameter;
  • the time of year;
  • the proportion of day-light hours in which the turbines operate;
  • the frequency of bright sunshine and cloudless skies (particularly at low elevations above the horizon); and,
  • the prevailing wind direction.

Only properties within 130 degrees either side of north, relative to the turbines can be affected at these latitudes in the UK – turbines do not cast long shadows on their southern side.
A105. The further the observer is from the turbine the less pronounced the effect will be. There are several reasons for this:

  • there are fewer times when the sun is low enough to cast a long shadow;
  • when the sun is low it is more likely to be obscured by either cloud on the horizon or intervening buildings and vegetation; and,
  • the centre of the rotor’s shadow passes more quickly over the land reducing the duration of the effect.

A106. At distance, the blades do not cover the sun but only partly mask it, substantially weakening the shadow. This effect occurs first with the shadow from the blade tip, the tips being thinner in section than the rest of the blade. The shadows from the tips extend the furthest and so only a very weak effect is observed at distance from the turbines.

A107. Problems caused by shadow flicker are rare. At distances greater than 10 rotor diameters from a turbine, the potential for shadow flicker is very low. The seasonal duration of this effect can be calculated from the geometry of the machine and the latitude of the site. Where shadow flicker could be a problem, developers should provide calculations to quantify the effect and where appropriate take measures to prevent or ameliorate the potential effect, such as by turning off a particular turbine at certain times.

A108. Careful site selection, design and planning, and good use of relevant software, can help avoid the possibility of shadow flicker in the first instance. It is recommended that shadow flicker at neighbouring offices and dwellings within 500m should not exceed 30 hours per year or 30 minutes per day.

A109. Turbines can also cause flashes of reflected light, which can be visible for some distance. It is possible to ameliorate the flashing but it is not possible to eliminate it. Careful choice of blade colour and surface finish can help reduce the effect. Light grey semi-matt finishes are often used for this. Other colours and patterns can also be used to reduce the effect further.