WIND TURBINE SHADOW FLICKER Prof. Gopu R. Potty, Ph.D. Department of Ocean Engineering University of Rhode Island Narragansett, RI 02882 firstname.lastname@example.org
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.
Shadow FlickerThe 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
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.
Shadow Plots1 Day 1 year
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
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)
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
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
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.
Existing Guidelines• Setback distances are generally not provided• Some guidelines requires interference to be considered and minimized.