Wind energy is rapidly growing in the United States and around the world. The growth of wind energy is stimulated by a desire to reduce carbon emissions from the electric power sector and reduce the effects of climate change. Projected industry growth and evolving turbine technology (i.e., taller turbines with larger rotor-swept areas and turbines with lower cut-in speeds) have heightened concerns about cumulative impacts on bat populations. In 2008, the Bats and Wind Energy Cooperative (BWEC) – an alliance of experts from government agencies, private industry, academic institutions, and non-governmental organizations – was formed to address these concerns. Since its formation, the BWEC has sought to develop and disseminate solutions to measure and mitigate the impacts of wind turbines on bats, while maintaining the ability to develop and operate wind energy facilities in a competitive and cost-effective manner. In 2008, the BWEC identified operational minimization, also referred to as curtailment, as a top priority for research. Since that time multiple studies testing the efficacy of various operational minimization techniques have been conducted. We assess the cumulative evidence of these studies by quantifying the efficacy of operational minimization using quantitative meta-analysis.
Publicly available studies that evaluated operational minimization across 19 treatments implemented at 8 wind energy facilities were summarized. These studies indicate that operational minimization is an effective strategy for reducing bat mortality at wind turbines and that the efficacy is measurable. We estimate that total bat fatalities are reduced by 33% with every 1.0 m/s increase in cut-in speed. Estimates of the efficacy of fatality reductions for every 1.0 m/s increase in cut-in speed on the three migratory tree-roosting species in North America are similar (28% for hoary bats [Lasiurus cinereus], 32% for eastern red bats [L. borealis], and 32% for silver-haired bats [Lasionycteris noctivagans]). Extrapolating these data across multiple facilities and years, a 5.0 m/s cut-in speed is estimated to reduce total bat fatalities by an average of 62% (95% CI: 54–69%). We estimate total bat fatality reductions at individual facilities in any given year to fall between 33%–79% (95% prediction interval). For individual species, average fatality reduction at 5.0 m/s cut-in speed was estimated as 48% (95% CI: 24%–64%) for hoary bats, 61% (95% CI: 42–74%) for eastern red bats, and 52% (95% CI: 30%–66%) for silver haired bats. Most variation in efficacy is attributed to inter-annual differences. The interannual differences in efficacy observed at the studies in our analysis outweighed any spatial difference in efficacy.
Several efforts that seek to improve the efficiency of operational minimization by minimizing both power loss and bat fatalities could not be included in this quantitative meta-analysis. We provide a narrative review of the current literature regarding other factors such as temperature, nocturnal timing, bat activity, and wind direction on improving the efficiency of operational minimization. We also review findings related to the reduction of fatalities due to feathering below the manufacturer’s recommended cut-in speed and on species of regulatory concern.
We conclude with a section summarizing the publicly available data regarding the loss in annual energy production (AEP) associated with operational minimization. Data on the loss in AEP is limited to five wind energy facilities across 11 operational minimization comparisons. The reported loss in AEP ranges from 0.06–3.20%, and is influenced by several factors, such as the prescribed cut-in speed, number of nights implementing operational minimization, turbine model, and wind regime. Additional considerations include, but are not limited to, the financial (e.g., market and price structure), technological (e.g., technology replacement and maintenance), and contractual liability (e.g., power purchasing agreements) associated with lost generation. Therefore, the loss of AEP is not the only constraint on the wind energy industry. Moreover, the circumstances at one site may have limited applicability to others and it is not appropriate to generalize cost or liability across facilities. Therefore, developing strategies that meet both economic and conservation goals is necessary.