Results of April – November 2018 Curtailment Evaluation, Acoustic Bat Monitoring, and Bird and Bat Carcass Surveys
The New Creek Wind Project (Project) began commercial operation in December 2016. This report summarizes results of curtailment evaluation, acoustic bat surveys, and bird and bat carcass monitoring that occurred at the Project between April and November 2018. This second year of monitoring was designed to evaluate the effectiveness and efficiency of a strategic curtailment strategy at reducing bat mortality, characterize conditions during which bats were active at nacelle height, and yield estimates of bird and bat fatality for the Project. The curtailment strategy was designed based on acoustic and weather data recorded at nacelle height throughout the 2017 monitoring period. Accordingly, year 2018 monitoring provided an opportunity to explicitly evaluate predictions made in 2017 regarding the cost and effectiveness of the curtailment strategy.
The 49 turbines at New Creek were divided into two operational groups for the 2018 monitoring period: 37 turbines were programmed to be feathered (blades pitched to prevent turbine rotation) under a strategic curtailment strategy based on site-specific bat data collected in 2017, and the remaining 12 turbines were to be feathered below the manufacturer’s cut-in speed of 3 m/s. The curtailment strategy raised the cut-in speed to 5.5 m/s between 1 April and 30 June, 6.0 m/s between 1 July and 30 September, 5.0 m/s during October, and 4.0 m/s between 1 and 15 November. Curtailment was implemented between sunset and sunrise, whenever temperatures exceeded 5 degrees C, during each seasonal period. Based on 2017 acoustic monitoring at nacelle height, we predicted that this strategy would protect 77% of bat activity from turbine operation. We predicted that the feathering-below-normal cut-in strategy would protect 20% of bat activity from turbine operation.
Curtailment criteria were met for an average of 5,170 out of 14,588 time periods (35%) at night between 1 April and 15 November among the 37 curtailed turbines. Turbine RPM was less than 1 for an average of
4,690 (91%) of the periods when conditions were met, indicating successful implementation of the curtailment plan. Wind speed was less than 3 m/s for an average of 1,583 out of 14,625 time periods (11%) at night between 1 April and 15 November among the 12 turbines that were to be feathered below 3.0 m/s. However, RPM was less than 1 for only 662 (42%) of these periods meeting shutdown criteria, and further analysis indicated similar turbine operation between day and night. Accordingly, these 12 turbines represented operational controls during the 2018 monitoring period.
Acoustic bat detectors were deployed at 10 of 49 project turbines and recorded a total of 12,051 bat passes during 1,356 detector-nights surveyed between 9 May and 15 November. Nine of the detectors operated properly for most of the monitoring period, while 1 detector (on Turbine 39) malfunctioned for most of the study period. Hoary bats (Lasiurus cinereus) accounted for 47% (n = 3,880) of recorded bat passes that were identified to species or species group (n = 8,315), with eastern red (Lasiurus borealis) and silver-haired bats (Lasionycteris noctivagans) accounting for 25% and 19% of identified passes, respectively. Only 9 passes were identified as Myotis species, occurring only at 2 detectors. Seasonal patterns in activity varied among species, with hoary bats and eastern red bats most active during August and silver-haired bats most active in September. Tri-colored bats (Perimyotis subflavus) accounted for only 2% of identified passes and were detected most often in August and September. Big brown bats (Eptesicus fuscus) were detected most frequently in late August and accounted for 4% of identified passes. Although species presence varied among nights, overall timing of bat activity showed similar patterns among detectors and species groups, with most activity occurring during the first few hours past sunset.
Bat activity showed clear relationships with temperature and wind speed measured at corresponding turbine nacelles, with 99% of passes for which weather data were available (n = 11,929) occurring when temperature was greater than 10° C and 61% of passes occurring at wind speeds less than 4.5 m/s. Considering temperature and wind speed together, bat activity occurred disproportionally during calm, warm conditions, and few bat passes were recorded during times with higher wind speeds or cooler temperatures. Also, results suggested an apparent interaction between the effects of temperature and wind speed on bat activity, with activity during windy conditions occurring primarily at warmer temperatures.
Curtailed turbines with operating bat detectors (n = 5) were effectively curtailed (conditions met and RPM < 1) during periods when 79 to 90% of recorded bat passes were detected. The control turbines with operating bat detectors (n = 4) were inactive (RPM < 1) during periods when 11 to 24% of bat activity occurred. Overall, the curtailment strategy protected 84.4% of bats recorded during the monitoring period, indicating that the strategy was more protective of bats than predicted based on 2017 data. Overall, 17.7% of bats detected at control turbines were not exposed to turbine operation.
Bird and Bat Carcass Monitoring and Fatality Estimates
Stantec searched all 25 odd numbered Project turbines at a weekly interval between 7 May and 14 November, conducting a total of 683 turbine searches during 135 days on-site. Individual turbines were searched on 24 to 28 occasions during the survey period. Searchers found 27 bat and 7 bird carcasses during standardized searches, and an additional 8 bat and 5 bird carcasses incidentally. Most carcasses were fresh (fatality estimated to have occurred the previous night), although searchers occasionally found carcasses estimated to be several days up to a week old. No federally listed bird or bat species were found during the survey period. Hoary bats (n = 20), eastern red bats (n = 8), and silver-haired bats (n = 4) accounted for 57%, 23%, and 11% of bat carcasses. Together, these long-distance migratory species accounted for 91% of bat carcasses, with big brown bats accounting for 6% of carcasses and tri-colored bats accounting for the remaining 3%. Bird carcasses represented 9 species, with no more than 2 of any single species found.
Ground conditions remained favorable for searching (short, sparse vegetation) throughout the monitoring period. Searcher efficiency was estimated to be 42% for bats and 67% for birds. Searcher efficiency estimates for bats were based almost entirely on mouse surrogates, which are typically more cryptic than bat carcasses, so we suspect that actual searcher efficiency for bats was higher, as documented in 2017 monitoring. Good ground visibility may have also contributed to the high scavenging rates documented throughout the monitoring period. Based on log-logistic model, which was most appropriate based on site-specific trials, carcass persistence was less than 1 day for birds and bats, with an estimated 16% of bat carcasses and 20% of bird carcasses persisting through the search interval. Taking into account searcher efficiency, carcass persistence, search interval, and density-weighted area correction factor, based on the Huso estimator, we obtained an estimated overall bird fatality rate of 3.19 birds/turbine (95% CI 1.90–5.34), combining the two operational groups (estimated bird fatality rates did not differ significantly between groups). Estimated bat fatality rate did differ significantly between operational groups, with a rate of 3.66 bats/turbine (95% CI 2.02–7.21) for curtailed turbines and 38.19 bats/turbine (95% CI 20.95–75.68) for control turbines. We included incidental carcasses found at search turbines in our estimates, and because bat searcher efficiency was likely higher than what was indicated by trials using mouse surrogates, we consider these fatality estimates to represent “worst case” scenarios.
The curtailment strategy in place between 1 April and 15 November 2018 prevented turbine operation during periods in which most acoustic bat activity (86% overall) occurred, resulting in an estimated bat fatality rate 90% lower than that for normally operating control turbines. Overall reductions in fatality were comparable to the amount of avoided bat activity. Our results suggest that the 2018 curtailment strategy effectively maintained low risk to bats throughout the monitoring period. The bat fatality estimate for 12 normally operating turbines was high, demonstrating substantial reductions in risk to bats associated with the curtailment strategy based on site-specific data collected in 2017.
Acoustic monitoring at nacelle height in 2017 provided valuable information for designing a smart curtailment strategy based on site-specific data and continued acoustic and carcass monitoring during 2018 verified the effectiveness of this curtailment program. Our results contribute to a growing body of evidence demonstrating that bat fatality can be maintained at low levels with strategic turbine curtailment, even in regions such as the mid-Atlantic, where risk to bats at wind farms is high. By incorporating multiple survey methods, the 2017 and 2018 monitoring periods provided a foundation of information and means to evaluate this approach to smart curtailment. Comparison of the 2017 and 2018 curtailment strategies indicates that lower cut-in speeds during early summer and late fall, combined with a lower overall cut-in speed during July–September allowed for additional power generation with little if any increased risk to bats.