Shadow Flicker: Living in Wind Energy’s Wake

The moving shadows caused by wind turbines, referred to as “shadow flicker,” are known to annoy some people. However, the relationship between the amount of exposure the the phenomenon and the annoyance it causes is poorly understood.

A new Lawrence Berkeley National Laboratory-led study published in Energy Research and Social Science examines the extent of shadow flicker exposure (measured in hours per year) around U.S. wind projects and identifies the key factors predicting its perception (awareness) and annoyance.

Researchers found that people are more likely to notice shadow flicker if they are exposed to it for longer and more numerous periods of time and less likely to notice the effect when exposure is low. Researchers found no clear relationship between the amount of exposure and self-reported annoyance.

Shadow flicker annoyance: Is it caused by frequency of exposure to it or other factors? Research led by Berkeley Lab identifies the key factors predicting shadow flicker awareness and annoyance.

Researchers from Berkeley Lab, Resource Systems Group (RSG), and Vermont Environmental Research Associates modeled shadow flicker at more than 35,000 residences located within 2 kilometers of wind turbines at 61 wind projects across 17 states.  Berkeley Lab collected in-depth perception and annoyance data from nearly 750 of these residences as part of a national survey of wind project neighbors.

“Our research found that the perception of shadow flicker is primarily a response to the quantity of exposure,” said Ben Hoen, a research scientist at Berkeley Lab and one of the paper’s authors. “Perceived shadow flicker is also influenced by other observable characteristics such as the distance to the nearest turbine and whether a respondent moved in after the project was built.”

Yet, the relationship between exposure and annoyance is much weaker. Study results indicate that annoyance is primarily a subjective response to other factors, such as wind turbine aesthetics and general annoyance to nearby sounds, like those from lawnmowers and traffic.

The researchers also found that local wind energy siting ordinances typically do not enforce shadow flicker exposure limits at all or do so ambiguously. Of the 50 U.S. counties represented in the study, most (62%) do not have any limits on flicker exposure. Of those that do have a limit, 30 hours per year is by far the most common; however, it’s unclear how the flicker is being modeled. This is important because the effect can be modeled in two ways that produce different results:

• “Worst case” — Assumes turbines are always operating, and no clouds are present to block them
• “Real case” — Assumes typical cloud cover and turbine operating characteristics, which is a typical European limit

In comparing both modeling methods, the researchers found that “worst case” modeling yields roughly 3 hours of shadow flicker for every 1 hour under “real-case” assumptions.

“Developers and local elected officials seeking to mitigate shadow flicker annoyance should not focus on modeled exposure alone,” said Ryan Haac of RSG. “A more transparent and inclusive planning process that allows local community members to have some influence over wind project aesthetics, for example, might be more productive.”

Courtesy of Wind Energy Technologies Office.