Abstract
Black Oak Wind, LLC is currently operating the 78-megawatt (MW) Black Oak Getty Wind Project (Project) in Stearns County, Minnesota (Figure 1). The Project became operational on December 23, 2016 and consists of 39 Vestas 2.0 megawatt (MW) V110 conical tubular steel tower wind turbine generators. Each turbine is a three-bladed, upwind, horizontal axis wind turbine with a rotor diameter of 361 feet and blades measuring 177 feet. Due to concerns regarding higher than anticipated bat fatalities in 2017, in 2018 turbines were split into two groups, a treatment group (20 turbines) and a control group (19 turbines), to assess the efficacy of a modified curtailment program. Treatment group turbines were feathered below manufacturer’s cut-in speed (3.0 meters/second [m/s]) every night from half an hour before sunset until half an hour after sunrise starting at midnight from April 1 through June 30, and were feathered up to 3.5 m/s from July 1 through September 30; these curtailment treatments occurred as long as the temperature during 10 minute rolling averages remained above 10 degrees Celsius (C; 50 degrees Fahrenheit). No feathering occurred for control group turbines from April 1 through September 30. Due to a programming error, the treatment and control groups received the opposite treatments from October 1 through 31, with no feathering occurring for the treatment turbines while the control turbines were feathered up to 3.0 m/s.
The second year of post-construction bird and bat mortality monitoring studies began on April 4, 2018, and continued through November 1, 2018. The objectives of the monitoring studies were to provide a summary of documented bird and bat fatalities, present estimates of searcher efficiency and carcass persistence, calculate annual fatality rates at per turbine and per MW levels, and assess the effectiveness of extending the period of feathering below manufacturer’s cut-in speed to the bat active period coupled with an increased cut-in speed between July 1 and September 30.
A total of four turbines had a search area of 120 meters x 120 meters cleared of vegetation and centered on the turbine; these cleared plots were searched twice per week. The gravel roads and pads at the remaining 35 turbines were searched within 60 meters of the turbine (the pads around turbines extended approximately 10 meters, and the access roads were searched out to 60 meters from the turbine); these road and pad searches occurred once a week during the study period. Searcher efficiency and carcass persistence trials were conducted throughout the study period using bat, small bird, large bird, and raptor carcasses.
A total of 43 bird carcasses comprising 24 unique species and seven unidentified carcasses were found during the study. The ruby crowned-kinglet and sedge wren were the two most commonly documented species (each with between three and four carcasses found). No obvious spatial or temporal patterns were detected in bird fatalities. One raptor carcass was found during the study: a Cooper’s hawk. While no federal- or state-listed species of birds were found as carcasses, two American white pelican carcasses, a species of special concern in Minnesota, were found during surveys. Using the Huso estimator, cleared plot estimates for small birds was 4.96 birds/MW/study period (90% CI of 1.08 - 9.58, using the Huso estimator) while the small bird road and pad estimate was 3.11 birds/MW/study period (90% CI of 1.59 - 4.91, using the Huso estimator). Estimates for large birds were roughly similar between the two search types, ranging between 0.25 birds/MW/study period (90% CI of 0.0 - 0.73) at cleared plots and 0.38 birds/MW/study period (90% CI of 0.11 - 0.79) at road and pads.
A total of 377 bat carcasses were found at the Project. Five species of bats were found: silver-haired bats (176 found), hoary bat (118 found), eastern red bat (48 found), big brown bat (31 found), and little brown bat (1 found). Additionally, three unidentified bats were found at the Project. Two special status bat species were documented; the big brown bat and the little brown bat are state species of special concern in Minnesota. No bat carcasses were found until late April. A noted increase in number of fatalities per turbine was observed from late July through September, and no bat carcasses were found after October 29. Across the Project, fewer bat carcasses were found per turbine in the northwestern portion of the Project compared to turbines in the southwestern, southeastern, and northeastern portions of the Project. The adjusted fatality rate estimate using the Huso estimator for cleared plots was 34.97 bats/MW/study period (90% CI of 1.82 - 83.02) for the control group and 26.67 bats/MW/study period (90% CI of 0.44 - 55.21) for the treatment group, and the road and pad estimate was 37.59 bats/MW/study period (90% CI of 28.06 - 53.72) for the control group and 21.00 bats/MW/study period (90% CI of 12.90 – 26.64) for the treatment group.
The curtailment strategy did appear to be effective in reducing bat mortality; the road and pad confidence interval did not overlap with the normal operation confidence interval for road and pad. Cleared plot estimates were higher for 2018 for both the control and treatment groups when compared to 2017; the estimate for 2017 on cleared plots was 13.03 bats/MW/study period (90% CI of 10.46 – 16.15). The road and pad estimate for treatment groups was lower than in 2017; the 2017 estimate for road and pad was 29.88 bats/MW/study period (90% CI of 17.81 – 49.10).