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Wind projects can offer positive economic and environmental benefits for communities and project owners but need to be sited carefully and with sensitivity to the surrounding landscape.  One of the concerns frequently voiced over wind power projects is shadow flicker.

What is Shadow Flicker?

Shadow flicker is a term used to describe the shadows cast by moving wind turbine blades in direct sunlight.  Depending on the sun’s position, the weather, the turbine rotor orientation, and other factors, this flickering shadow can sometimes fall upon an occupied structure or other sensitive area.  Shadow flicker is usually of concern only for larger wind turbines, such as the multi-megawatt turbines used in community and commercial scale projects.  Smaller wind turbines, such as those used for residential applications, generally do not create noticeable shadow flicker impacts.

Is Shadow Flicker Dangerous?

There is no evidence that shadow flicker causes any harmful health effects.  In everyday life, such as driving a car, most people experience flickering shadows.  Shadow flicker is problematic because it may be considered an annoyance by the occupants or users of an area where it occurs.

How is Shadow Flicker Assessed?

Shadow flicker is typically expressed in terms of duration, usually in terms of hours of impact per year.  Calculating this impact involves a number of factors:

  • Sun position
  • Wind direction/rotor orientation
  • Cloudiness and weather
  • Location of sensitive receptors
  • Elevation
  • Terrain/obstructions
  • Turbine/tower specifications

Industry software packages such as WindPro, when used properly, are able to make these calculations and generate predictions of the shadow flicker impacts for the area surrounding the proposed turbine.  An example graphic generated by this kind of analysis is shown in the figure below.

This shadow flicker analysis reveals the anticipated impact on the area surrounding a proposed turbine.

The butterfly-like shape is made up by lines of shadow flicker impact, similar to the lines on a topographic map.  For example, a house with windows facing the proposed turbine located along the blue line could expect about 30 hours of shadow flicker in a typical year.

Are There Standards for Shadow Flicker Impacts?

Unfortunately, most zoning and other regulations related to wind power only refer to shadow flicker in a general sense, using language such as “shall generate no significant impact.”  Some statewide and international standards define “significant impact” as 30 hours or more of shadow flicker per year.

Can Shadow Flicker be Mitigated?

There are several ways to address potential shadow flicker impacts.  The best method is to have a shadow buffer zone between the proposed turbine and any residences or other sensitive receptors.  If that is not possible, shadow flicker can be mitigated by planting trees or other visual screens between the affected receptor and the turbine.  Finally, the project owner can program the turbine to shut down during certain times of day, or under certain conditions, to reduce shadow flicker impacts.

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