Page 292 - Fluid Power Engineering

258 Chapter Twelve

Shadow Flicker
During sunrise and sunset, wind turbine blade casts a shadow that
alternates as blades cover the sun for a short period. This is called
shadow flicker and is relevant only if there are houses or other build-
ings with windows on which a shadow falls. The shadow flicker is
pronounced on buildings that are less than 1,000 ft from a turbine. 1
The shadow flicker happens only during sunrise and sunset for a few
minutes each day. Wind assessment applications, like WindPRO and
WindFarmer, have tools to assess the impact of shadow flicker. In ad-
dition to summary numbers like 20 h of flicker annually, they also
provide detailed location-by-location and day-by-day assessment of
start time and end time of shadow flicker.
Only Germany has guidelines for calculating shadow flicker and
an allowable limit on the amount of shadow flicker. The calculation
8
guidelines are: (a) The angle of the sun over the horizon must be at
least 3 ; (b) the blade of the wind WTG must cover at least 20% of the
◦
sun.
The maximum shadow impact for a neighbor to a wind farm ac-
cording to the German guidelines is: (a) Maximum 30 h per year of
astronomical maximum shadow (worst case); (b) maximum 30 min
worst day of astronomical maximum shadow (worst case); (c) if au-
tomatic regulation is used, the real shadow impact must be limited to
8 h per year.
The known impact of shadow flicker is to epilepsy patients. Ac-
cording to the Epilepsy Foundation, frequencies above 10 Hz are likely
1
to cause epileptic seizures. For most utility-scale turbines, the fre-
quency of rotation is 0.6 to 1 Hz, so the frequency of flicker for a
three-blade turbine is 1.8–3 Hz.

Aesthetic Impact
Visual impact is the most difficult to quantify and is often the primary
reason for opposing a wind project. Although there is widespread sup-
port for wind energy projects, in general, the support drops when the
project is close to one’s community because of the not-in-my-backyard
(NIMBY) syndrome. Most projects do not adequately address visual
impact issues and even when visual assessments are done, subjective
approaches are adopted to assess visual impact. In this section, a few
established methods will be covered for visual assessment.
NRC has outlined a detailed assessment process for evaluating
the visual impact of wind projects. It is a six-step process: 1

1. Project description. All relevant wind project data is col-
lected in this step including: Turbine details like size, color,