When siting a wind turbine in a populated area, one of the issues that needs to be addressed is shadow flicker. In the early days of the wind energy industry in the U.S. this was not an issue as projects were being installed in remote areas. As wind energy use has increased and wind turbines are being installed closer to where people live and work –more like they are in Europe – the issue has become more important.
Shadow flicker is the shadow created by moving wind turbine blades. Since the blades can be 140 feet long or more and perched on a tower 200 feet high or more, and the sun may be at a low angle, the shadow can also be long and distant from the base of the tower. Here in the mid latitudes of the northern hemisphere, the sun will always be south of the zenith. That means the shadow will be north of the tower except near sunrise and sunset on summer days.
The shadow of a moving blade on a residence can be a significant annoyance. At a recent meeting of the Rhode Island Renewable Energy Siting Partnership, a couple living near the Portsmouth High School turbine showed a video of the shadow flicker at their house. To me, it looked like someone turning a bright light off and on over and over. It was clearly an annoyance. The key factor seems to be how many hours a year such shadows occur. For some Portsmouth neighbors of a wind turbine where the shadows only occur infrequently, the homeowners don’t seem to mind. With long durations, it’s annoying.
For drivers, moving shadows are common. Other vehicles – on the same road or on overpasses – cause moving shadows. A car is constantly moving in and out of shadows from trees, bridge structures, trucks, buildings, etc.
There are computer programs that can map the area where shadow flicker will occur. The Jamestown Wind Energy Committee had its contractor evaluate shadow flicker in its comparison of potential sites on Jamestown. Taylor Point scored well. The Taylor Point shadow flicker analysis is shown in Figure 7-2. The green area shows where shadows will fall with longer durations nearest the wind turbine and shorter durations farthest from the wind turbine. The key results for Taylor Point are:
• Most of the shadows fall on the water.
• No residence will be affected.
• Drivers heading east from the toll booth early on mid-summer mornings will see moving shadows for a couple seconds as they head for the bridge.
• No drivers on the bridge will be affected.
• Mechanics working in the Rhode Island Turnpike and Bridge Authority garage very early on summer mornings will see a few moments of moving shadows.
As the Wind Energy Committee concluded, Taylor Point does well from a shadow flicker standpoint.
The author is president of Endless Energy Corporation, a consultant chosen by the Town Council in December to conduct studies necessary to determine the costs and potential profitability of a Jamestown wind turbine.
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