A review of the effectiveness of operational curtailment for reducing bat fatalities at terrestrial wind farms in North America

Abstract

Curtailment of turbine operations during low wind conditions has become an operational minimization tactic to reduce bat mortality at terrestrial wind energy facilities. Site-specific studies have demonstrated that bat activity is higher during lower wind speeds and that operational curtailment can effectively reduce fatalities. However, the exact nature of the relationship between curtailment cut-in speed and bat fatality reduction remains unclear. To evaluate the efficacy of differing curtailment regimes in reducing bat fatalities, we examined data from turbine curtailment experiments in the United States and Canada in a meta-analysis framework. We used multiple statistical models to explore possible linear and non-linear relationships between turbine cut-in speed and bat fatality. Because the overall sample size for this meta-analysis was small (n = 36 control-treatment studies from 17 wind farms), we conducted a power analysis to assess the number of control-treatment curtailment studies needed to understand the relationship between fatality reduction and change in cut-in speed. We also identified the characteristics of individual curtailment field studies that may influence their power to detect fatality reductions, and in turn, contribute to future meta-analyses. We found strong evidence that implementing turbine curtailment reduces fatality rates of bats at wind farms; the estimated fatality ratio across all studies was 0.37 (p < 0.001), or a 63% decrease in fatalities. However, the nature of the relationship between the magnitude of treatment and reduction in fatalities was more difficult to assess. Models that represented the response ratio as a continuous variable (e.g., with a linear relationship between the change in cut-in speed and fatalities) and a categorical variable (to allow for possible non-linearity in this relationship) both had substantial support when compared using AICc. The linear model represented the best fit, likely due to model simplicity, but the non-linear model was the most likely without accounting for parsimony and suggested fatality rates decreased when the difference in curtailment cut-in speeds was 2m/s or larger. The power analyses showed that the power to detect effects in the meta-analysis was low if fatality reductions were less than 50%, which suggests that smaller increases in cut-in speed (i.e., between different treatment categories) may not be easily detectable with the current dataset. While curtailment is an effective operational mitigation measure overall, additional well-designed curtailment studies are needed to determine precisely whether higher cut-in speeds can further reduce bat fatalities.

Timing and Weather Offer Alternative Mitigation Strategies for Lowering Bat Mortality at Wind Energy Facilities in Ontario

“…Turbines operate for half the time when the curtailment wind speed is the same as the median; although the result is a reduction in mortality risk, it does not indicate a causal relationship between bat activity or mortality and 5.5 m/s wind speed. We note that more recent studies have shown a consistent reduction in bat mortality for every 1 m/s increase in cut-in speed (50% reduction in [ 23 ], 33% reduction in [ 20 ]). A 40–60% reduction in mortality would be an outcome consistent with mortality being reduced because the turbines are curtailed about half the time.…”

“…Curtailment based on wind speeds reduces, but does not eliminate, bat mortality at wind energy facilities [ 19 , 20 ]. Experimental studies show that reductions in mortality range from about 50–70% (e.g., 50–70% in Alberta [ 16 ]; 47% in West Virginia [ 21 ]; 62% in Vermont [ 22 ]; 63% throughout North American wind farms [ 23 ]).…”

Offshore Occurrence of a Migratory Bat, Pipistrellus nathusii, Depends on Seasonality and Weather Conditions

“…High-risk areas are coasts and forested hills, whereas mortality is relatively low in agricultural areas further away from the coast [ 7 ]. Effective mitigation has been achieved by curtailment procedures that limit the production time of wind turbines [ 16 , 17 , 18 , 19 , 20 ]. Increasing the cut-in speed at which blades start rotating to wind speeds of 5 m/s has resulted in an average reduction of 62% in the number of fatalities [ 20 ].…”

Curtailment and acoustic deterrents reduce bat mortality at wind farms

“…Reducing bat mortality at wind‐energy projects is of growing importance given the potential impacts of wind energy on tree bat populations (Frick et al 2017). Curtailment at wind speeds below 5.0 m/second reduces bat mortality from 42–78% (Arnett et al 2013 b , Hein et al 2014, Martin et al 2017, this study), and some evidence exists that greater reductions in bat fatalities can be achieved by curtailing operations at higher wind speeds (Good et al 2012, Arnett et al 2013 b , Adams et al 2021). Curtailment at higher wind speeds results in greater losses in energy production and may not be economically feasible at many wind‐energy projects.…”

“…Wind turbine curtailment, also known as blade feathering, is a strategy that prevents turbines from spinning at night at speeds >1–2 rotations/minute, when wind speeds are below a pre‐determined value. Wind turbine curtailment reduces fatalities of nearly all bat species affected by wind turbines (Arnett et al 2013 b , Martin et al 2017, Adams et al 2021). Curtailment (i.e., cut‐in speed curtailment) results in lost energy production at night and causes adverse impacts on wind project economics if not optimized.…”