Draft National Wind Farm Development Guidelines - July 2010

E
Shadow Flicker
E.1 Introduction
This Appendix discusses the shadow flicker phenomenon and presents a best practice
methodology to assess and mitigate against its affects. Specifically, the Appendix
provides:
• An explanation of the nature of shadow flicker.
• Consideration of suggested impacts of shadow flicker, identifying impacts with
negligible risk and impacts with significant risk.
• An explanation of the shadow flicker assessment method with practice note;
• Linkage between the shadow flicker assessment method and the phases of wind farm
development and operation.
• The purpose of this Appendix is to provide the technical detail necessary to:
• Explain the phenomenon of shadow flicker, its causes and consequences.
• Specify wind farm design methods necessary to minimise the impact of shadow flicker
on wind farm stakeholders.
E.2 Background
E.2.1
What is shadow flicker
The rotating blades of wind turbines can cast intermittent shadows that appear to flicker
for an observer at a fixed ground position. Since wind turbines are tall structures, shadow
flicker can be observed at considerable distances but usually only occurs for brief times at
any given location. The most common effect of shadow flicker is annoyance. These
impacts are most closely associated with the duration of shadow flicker experienced
above a certain intensity.
The duration of shadow flicker, its intensity and the locations it affects are most strongly
determined by the relative position of the Sun, the turbine, and the receptor. The relative
position of the Sun varies with latitude, time of day and time of year. Other influential
factors include:
• The size of the wind turbine rotor and height of the tower
• Surface topography
• Intervening vegetation
• Direction of the wind (and hence the rotor plane of the wind turbine)
• Weather (particularly cloud cover)
• General visibility (including presence of mist, smoke and other particulates)
The typical effect of shadow flicker for a single turbine is shown in Figure E-1. This figure
shows a plan view of a typical turbine with associated areas affected by shadow flicker.
The following features are exhibited:
• The different colour bands represent different annual exposure to shadow flicker (blue
0-10 hr/yr, green 10-20 hr/yr etc)
• The areas (directions) affected at different times of day and year are shown
• The zone within which shadows are likely to be visible is shown. Intensity decreases
with distance and only shadows occurring within a certain distance of a turbine are
likely to be visible
Draft National Wind Farm Development Guidelines – 2 July 2010 Page 147