1. WIND TURBINE SHADOW FLICKER
Prof. Gopu R. Potty, Ph.D.
Department of Ocean Engineering
University of Rhode Island
Narragansett, RI 02882
potty@egr.uri.edu

2. Shadow Flicker
• occurs when the sun passes behind the rotors of
and casts a shadow over neighboring properties.
• As the blades rotate, the shadow flicks on and off,
an effect known as shadow flicker.

3. Shadow Flicker
The likelihood and duration of the effect depends upon:
• Direction of the property relative to the turbine
• Wind speed and direction
• Distance from turbine
• Turbine height and rotor diameter
• Time of year and day
• Weather conditions (i.e. cloudy days)
http://www.currentresults.com/Weather/Rhode‐Island/annual‐weather‐averages.php

4. Shadow flicker is most pronounced in
northern latitudes during winter months
because of the lower angle of the sun in
the winter sky.
Shadows cast close to a turbine will be
more intense, distinct and focused.

5. Shadow Plots
1 Day 1 year

6. Health Concerns
• Epileptic seizures
• typically by light flashes 5 to 30 Hz;
Wind turbine flicker if less than 1 Hz
• Eye strain, headaches, nausea and
disorientation
• mostly anecdotal evidence
• Nuisance ‐ intrusive and annoying

7. EXISTING GUIDELINES AND REGULATIONS
Guideline or Regulation
Germany 30 hr/year or 30 min/day
Denmark 10 hr/year
Netherlands 17 days/year or 20 min/day
Massachusetts No Limit “minimize flicker”
Maine No Limit “avoid unreasonable shadow flicker”
New Hampshire 30 hr/year
Ohio 30 hr/year
Wisconsin 30 hr/year (mitigation required if greater than 20 hrs/year)

8. Signal Interference
• Signal blocking:
– behind the turbine for a limited distance creating a
shadow zone
– Shadow zone depends on
• material, geometry (height and width) of turbine
• Signal Reflection
– Reflection when structure is in line of sight to a
transmitter
– Reflection depends on
• material, rotational speed of turbine, geometry and
orientation of blades relative to transmitter

9. Technologies Affected
• Television (ghosting)
– Lower problem for digital signals
• Satellite Television
– Rarely a problem since signals are received from
very high
• FM and DBA radio:
– Interference possible only within few 10’s of
meters
• Scanning telemetry systems:
– Work in the UHF band and hence susceptible to
multi‐path effects from reflecting blades.
• Fixed radio links:
– Public safety radio systems work using microwave
wavelengths can be affected when the wind
turbine is placed within the line of sight between
the transponder and a receiver

10. Cellular Phones
• EM noise from turbines
– insignificant
– created is outside the cell phone band
• Near field zone of transmitting and
receiving antennas is approx. 20 m.
– Objects within this zone can conduct or
absorb radio waves.
• Diffraction: Partial or total blockage of
signal resulting in lower signal strength.
– Problem when any object is within the first
Fresnel zone
• Reflection: Caused when an object reflects
the waves.
– Less of a problem for blades made of glass
reinforced plastic and when the turbine is
separated from the tower by more than
325 ft.

11. Existing Guidelines
• Setback distances are
generally not provided
• Some guidelines requires
interference to be
considered and minimized.