Abstract
The potential effects of climate change on wildlife are vast, multifaceted, and well represented in scientific literature. Rapid expansion of renewable energy infrastructure is a key part of any global strategy to reduce the pace and severity of anthropogenic climate change, although the potential impacts of renewable energy infrastructure on wildlife are also becoming increasingly apparent. Bats appear vulnerable to population-level impacts from the cumulative effect of turbine-related fatalities at commercial wind energy facilities in North America, particularly as the industry continues to expand to meet renewable energy generation targets. Long-distance migratory species account for the largest proportion of bat fatalities documented in North America, and current mortality rates could threaten the viability of hoary bats (Lasiurus cinereus) in particular. Fatalities of federally listed and candidate species including Indiana bats (Myotis sodalis), northern long-eared bats (Myotis septentrionalis), gray bats (Myotis grisescens), and tricolored bats (Perimyotis subflavus) also occur at wind energy facilities, necessitating measures to minimize risk and metrics to validate the success of such measures.
Turbine curtailment is the most widely used and consistently effective method to reduce bat fatality rates and involves pitching turbine blades parallel to prevailing winds to restrict turbine rotation when turbines would otherwise be operating and capable of producing power. Curtailment is effective because it eliminates the exposure of bats to fast-moving turbine blades but is unpopular with the wind energy industry due to the resulting energy loss and associated cost. The higher the cut-in speed wind speed threshold below which turbines are curtailed, the greater the reduction in risk to bats and the greater the associated amount of energy loss, although the actual cost of curtailment is determined by weather conditions and wind regime and is therefore difficult to predict.
If conducted with sufficient intensity, carcass monitoring can show that curtailment reduces bat fatality estimates, but the precision and accuracy of fatality estimates are limited by small sample sizes of bat carcasses, imperfect detection, carcass removal, and incomplete coverage relative to the area in which carcasses may fall. More importantly, carcass data do not indicate precisely when fatalities occurred and therefore provide coarse feedback on wind speed and temperature when bats were at risk. Lacking site-specific information to guide when and under what conditions curtailment should be applied, most curtailment strategies apply a single cut-in speed, typically selected by regulatory precedent rather than site-specific data. There is seldom an opportunity or desire to adjust parameters of these so-called “blanket” curtailment strategies due to the lack of information to guide such adjustments and the high cost of subsequent carcass monitoring that would be required to determine if the changes achieved the intended effect. The combination of these factors has severely limited the ability to use curtailment strategically as a tool to manage risk to bats.
The rapid expansion of the wind energy industry coupled with increasing awareness of the potential impacts of cumulative turbine-related fatalities on bat populations highlights the need to understand and manage environmental turbine-related impacts to bats more aggressively and strategically than the current use of blanket curtailment allows. “Smart” curtailment strategies offer a potential solution by concentrating curtailment on periods and conditions where bats are most active, thereby protecting bats while simultaneously reducing the overall amount of curtailment and associated energy loss. Smart curtailment can be implemented using a variety of methods, but strategies share a common principle that curtailment is only beneficial when bats are present in the rotor-swept zone; curtailing at other times does nothing to reduce risk but results in energy loss all the same. The challenge for implementing smart curtailment is knowing when bats are likely present or absent in the rotor-swept zone.
Regardless of what type of curtailment is implemented and how strategies are designed, the wind energy industry and regulators alike need a better method than carcass searches to measure curtailment effectiveness and tailor curtailment strategies around patterns in risk. Through a combination of existing data summary and analysis and an unprecedented deployment of turbine-mounted acoustic bat detectors at wind energy facilities across Iowa, this study demonstrates that acoustic exposure (the rate or proportion of bat passes detected when turbine rotors are moving) is an ideal metric to evaluate the effectiveness of curtailment strategies and provides feedback on how to optimize curtailment strategies to balance the simultaneous needs to boost renewable energy generation and reduce cumulative impacts to bats.
This study spanned a period of nearly five years, beginning with an initial demonstration, based on data collected previously at a pair of wind energy facilities in West Virginia, that acoustic exposure and bat fatality rates were positively correlated across multiple timescales (Peterson et al. 2021). Building on this successful proof-of-concept, we conducted an intensive acoustic monitoring effort to supplement ongoing bat carcass monitoring at the Orient and Arbor Hill wind energy facilities in Iowa in 2021–2023; this effort was expanded in 2022–2023 to additional facilities throughout Iowa, with acoustic monitoring occurring at a total of 210 turbines at 13 wind energy facilities across the state. The overall purpose of this research project was to demonstrate how acoustic data from turbine-mounted bat detectors could be used to design curtailment strategies that achieve equivalent reduction in bat fatality while resulting in less energy loss and subsequently measure the effectiveness of these strategies. This report is arranged around four research objectives identified in our study plan, each of which is addressed in a separate section.
The first objective was to characterize the seasonal and temporal distribution of bat activity and explore relationships with temperature and wind speed, then evaluate the consistency of such patterns among facilities, turbines, species, and years. Seasonal distribution of bat activity across the 13 wind energy facilities monitored was consistent, aligning with well-established patterns that have also been observed in pre-construction acoustic surveys and fatality monitoring results throughout much of North America. Seasonal distribution varied somewhat among species, but most bat activity occurred between mid-July and early September across species. Of the variables we considered, day of year (a proxy for season) and time of night were the most important factors in predicting bat presence in the rotor-swept zone, followed by wind speed and temperature, with turbine and year ranked least important at most facilities. In the context of designing curtailment strategies to reduce fatality risk to bats, these results suggest that the broad seasonal and temporal patterns in acoustic bat activity in the rotor-swept zone provide a reliable basis for designing activity-based curtailment strategies that apply higher cut-in speeds (and as a result, more curtailment) when bats are most active and lower cut-in speeds when bats are less active.
The second objective was to quantify the relationship between exposed bat activity and fatality, both in terms of alignment between fatality estimates derived from carcass searches against acoustic exposure measured acoustically and in terms of the ability to discern the effect of curtailment using acoustic data versus carcass data. Reanalysis of acoustic data and fatality estimates from a pair of wind energy facilities in West Virginia demonstrated a close relationship between acoustic exposure and fatality and culminated in a peer-reviewed paper cited herein (Peterson et al. 2021) and an interim technical report, included as Appendix A. Fatality estimates at the 13 facilities in Iowa, whether based on weekly road and pad carcass searches or twice-weekly searches of cleared plots, showed no discernable relationship with acoustic exposure. The level of effort for carcass searches used in this study was designed to satisfy permit compliance requirements when aggregated among sites but was insufficient to measure fatality rates with the accuracy needed to differentiate operational treatments due to small sample sizes of carcasses and uncertainty introduced by bias correction factors. The lack of correlation between acoustic exposure and fatality estimates in our study does not mean these processes are not related but illustrates the challenge in evaluating curtailment strategies using carcass searches when sample sizes are limited. In the second part of this objective, fatality estimates did not indicate consistent effects of curtailment, whereas curtailment resulted in clear and consistent reductions in acoustic exposure relative to operational control treatments across facilities, years, and even among individual turbines. The resulting rate of acoustic exposure (exposed passes per detector night) varied among sites, but proportional reductions in exposure due to curtailment were similar among facilities and years, suggesting that the magnitude of risk to bats varied among sites in our study but that curtailment reduced risk by a relatively consistent margin. Acoustic exposure also provided accurate, quantitative feedback on how successfully curtailment was implemented per facility, treatment, and turbine, highlighting a practical advantage of using acoustic exposure to evaluate curtailment.
Our third objective was to demonstrate use of nacelle-height acoustic bat and weather data to optimize site-specific smart curtailment strategies. We used acoustic data recorded at two facilities in 2021 to design a smart alternative to blanket curtailment below 5.0 meters per second (m/s); this alternative was then implemented at subsets of turbines at two facilities in 2022 and at five additional facilities in 2023. The smart curtailment alternative successfully reduced acoustic exposure by the same margin as blanket curtailment while resulting in less energy loss at almost all sites where implemented. We also demonstrated the ability to simulate turbine operation using wind speed and temperature data, enabling acoustic exposure to be evaluated for each of the three operational treatments (operational control, blanket, and smart curtailment) as if they had been implemented across all facilities in our study. Simulated exposure improved the ability to measure inter-facility and inter-year comparison across a larger sample size and represents a key advantage of acoustic exposure measurement in assessing curtailment effectiveness. Our results demonstrated that slight changes in cut-in speed and start/stop times in designing the smart curtailment alternative, though based on data from only 2 sites, effectively yielded equivalent levels of exposure reduction and less energy loss than blanket curtailment when applied to 13 facilities across Iowa over multiple years.
The final objective of this study was to compare effectiveness and energy loss between blanket and smart curtailment programs. Building on the results of the third objective, we simulated four blanket curtailment strategies with cut-in speeds ranging from 5.0–8.0 meters per second (m/s) and designed a smart curtailment alternative for each, then compared acoustic exposure and energy loss across each pair of strategies across facilities and years. The smart curtailment alternatives successfully outperformed blanket alternatives across facilities and among years in almost every case, highlighting the consistency in bat activity patterns around which smart curtailment alternatives were tailored. We demonstrated an exponential increase in the difference in energy loss between blanket and smart curtailments alternatives at higher cut-in speeds. As was the case for the blanket strategy implemented in the study, curtailment strategies resulted in similar reductions in the percent of acoustic exposure, while the cumulative rate of exposed passes was more variable among sites. This highlights an opportunity to use curtailment strategically to manage risk across a fleet of wind energy facilities by adjusting cut-in speeds or otherwise scaling the intensity of curtailment according to the site-specific level of risk.