Background
Bat mortality at wind facilities is most frequently reported during the fall and mainly consists of tree bat species: eastern red bat, silver-haired bat, and hoary bat. Bats also tend to be more active during lower wind speeds, and this time is when mortality risk is at its highest.
One way of reducing bat mortality at wind farms is to curtail (stop) turbine blades from spinning when wind speeds are below a certain threshold (e.g., 7 meters/second). This technique reduces bat mortality, but it also reduces the energy generation of the wind farm. Unfortunately, this approach curtails turbines regardless of if bats are present, leading to unnecessary power generation loss. A better approach is to curtail turbines only when bats are present and wind speeds are within the curtailment zone, thereby minimizing mortality while also maximizing operational time. Using bat presence as a deciding factor for curtailment, along with other environmental variables, is called smart curtailment.
In collaboration with EPRI, the following study represents a test of the Turbine Integrated Mortality Reduction (TIMR) system at the Blue Sky Green Field Wind Farm in Wisconsin. This controlled study assessed the effectiveness of the TIMR system by comparing mortality rates at turbines where TIMR was operational with those at turbines where TIMR was not operational (i.e., turbines were operating normally).
The TIMR Smart Curtailment Technology
The TIMR system consists of three main components that provide a wind farm with smart curtailment:
- Bat acoustic detectors placed on the turbine nacelle to detect bats in the rotor-swept zone
- A TIMR server that processes bat acoustic data and wind speed data from the facility
- A SCADA interface to shut down the turbines if bats are present and winds are within the curtailment zone
All three of these components are custom-configured for each wind farm based on the results of a feasibility study of the wind regime and bat activity information garnered from acoustic monitoring or post-construction fatality data. While these data are not required to use a TIMR system, they are helpful for better informing curtailment parameters before beginning a smart curtailment strategy. Additionally, our TIMR system can meet a wind farm’s stringent cybersecurity requirements, which can affect remote access to TIMR hardware. Great care is taken to work closely with wind facility staff to ensure that all security and logistical requirements are met.
TIMR Technology Testing
We deployed the TIMR system at the Blue Sky Green Field wind farm from June 1, 2015, to October 31, 2015. We placed four bat detectors across the wind farm to sample the airspace above and below the nacelle. All audio files were subjected to an algorithm to detect probable bat calls. Bat detectors sent all probable call files to the TIMR server at the wind farm’s operations center, where acoustic data were processed in near real-time to inform turbine shutdown. Wind speed data were collected in 10-minute increments and provided by the SCADA system to the TIMR server. The average wind speed for each 10-minute period was used as the wind speed measurement when deciding on a turbine shutdown.
The binary TIMR risk model uses 0 to indicate low risk (no curtailment) and 1 to indicate high risk (smart curtailment). High risk was determined if the measured wind speed was <8.0 m/s and ≥ 1 bat was detected in the previous 10 minutes: Under these conditions, the turbines monitored by TIMR were curtailed; all control turbines operated normally. When high-risk conditions were identified, the turbines were curtailed for 30 minutes. After 30 minutes, the risk model reevaluated the conditions every 10 minutes and continued to do so until low-risk conditions were identified. If the wind speed was ≥ 8.0 m/s, all turbines operated normally. Ten control and ten treatment turbines were searched once daily for bat fatalities during the June 1-October 31 study period. All plots were mowed and treated with herbicide to ensure carcasses were visible to observers.
For all species, the number of fatalities found at TIMR turbines (3-9 carcasses per species) was less than at control turbines (fully operational; 27–48 carcasses per species). After accounting for searcher efficiency and carcass persistence, the number of estimated fatalities was significantly higher at control turbines than at TIMR turbines.
Overall, our study estimates that the use of TIMR decreased fatalities by 85% across all species and between 74% and 91% for individual species compared to control turbines (Table 1). The annual reduction in power generation and revenue was 3.2% compared to control turbines. These results are similar to what was observed in other studies that examined the effects of curtailment, but these didn’t use the presence of bats to inform the curtailment decisions. Note that these results presented here only apply to the Blue Sky Green Field wind farm and the specific configuration and curtailment parameters used. Using different cut-in speeds and/or curtailment thresholds could change the results significantly. Further research will indicate the extent to which adjusting TIMR configuration parameters will affect the fatality estimates.
Table 1. Percent mortality reduction with the TIMR system compared to normal operation.
A more recent study is currently being reviewed. Be on the lookout for a Part 2 with more current data soon!
References
Arnett, E. B., Huso, M. M., Schirmacher, M. R., & Hayes, J. P. (2011). Altering turbine speed reduces bat mortality at wind energy facilities. Frontiers in Ecology and the Environment 9:209–214. https://esajournals.onlinelibrary.wiley.com/doi/10.1890/100103
Arnett, E. B., Baerwald, E. F., Mathews, F., Rodrigues, L., Rodriguez-Duran, A., Rydell, J., Villegas-Patraca, R., & Voigt, C. C. (2016). Impacts of wind energy development on bats: a global perspective. In C. C. Voigt and T. Kingston(Eds.), Bats in the Anthropocene: conservation of bats in a changing world (pp. 295-323). Springer International Publishing, New York, New York, USA. https://link.springer.com/chapter/10.1007/978-3-319-25220-9\
Frick, W. F., Baerwald, E. F., Pollock, J. F., Barclay, R. M. R., Szymanski, J. A., Weller, T. J., Russell, A. L., Loeb, S. C., Medellin, R. A., & McGuire, L. P. (2017). Fatalities at wind turbines may threaten population viability of a migratory bat. Biological Conservation 209:172–177. https://www.sciencedirect.com/science/article/abs/pii/S0006320716310485?via%3Dihub
Hayes, M. A., Hooton, L. A. Gilland, K. L., Grandgent, C., Smith, R. L., Lindsay, S. R., Collins, J. D., Schumacher, S. M., Rabie, P. A., Gruver, J. C. & Goodrich-Mahoney, J. (2019). A smart curtailment approach for reducing bat fatalities and curtailment time at wind energy facilities. Ecological Applications 00(00):e01881:10.1002/eap.1881. https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1002/eap.1881
