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Exploring the Potential Impacts on Birds and Bats from Taller Turbines

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Mike Morgante's picture
Technical Operations Manager Ecology and Environment, Inc.

Management of Buffalo office technical staff; mentoring and employee development; qualityassurance; knowledge transfer. Project Director / Manager of Avian and Bat Studies, NEPA...

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  • Jun 20, 2019 6:30 am GMT

This item is part of the Special Issue - 2019-06 - US Wind Power, click here for more

As the sizes of wind turbines increase, we can expect some differences in the potential impacts on birds and bats. A few representative models showing the evolution toward taller turbines are shown in the figure below. Not only are they taller, but there is a larger rotor sweep. The hub height determines where the maximum and minimum reaches are for the rotor, and thus where there could be impacts to birds and bats. Older turbines, such as the GE 1.5 MW, with a hub height of 80 meters and a 77-meter rotor diameter, and current turbines with a hub height of 110 meters and 137- meter rotor diameter (such as the GE 3.6 MW) can have the same height for the lowest reach of the rotor, but substantially different maximum height and rotor sweep sizes. Future turbines will continue to increase, with the Siemens Gamesa 5.8 MW, for example, reaching 135 meters of hub height and a 170-meter rotor diameter.

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Compare those heights with typical avian flight altitudes, which vary considerably by many factors including type of bird, topography, geography, and weather. For example, most diurnal bird flight paths go up to 150 meters while most nocturnal songbird migrant flight paths go between 120 to 600 meters. In general, taller turbines have a larger rotor sweep in a zone where more nocturnal migrant birds fly, which could result in more collisions per turbine. As diurnal migrant raptors already have a high degree of detection and avoidance, taller turbines are less likely than topography, geography, and weather to influence potential collisions. In some cases, taller turbines will have higher rotor sweep than some lower flying diurnal bird species. For example, the bobolink usually flies close to the ground for its flight song on breeding ground from 2 meters up to 40 meters, below many sweep zones, while the horned lark flight song at 80 meters to 250 meters is still in sweep zones and thus at higher risk.

Regarding bat collisions, the impacts are mostly speculative and in need of study for modern taller turbines. Some older studies showed increase in fatality rates with taller turbines, but those turbines are small by today’s standards, while other studies showed no correlation with turbine height. The impacts are expected to vary by group, if not species of bat. Those with lower foraging heights are likely to benefit from higher rotor sweeps, such as the Myotis bat species that are smaller and weaker flyers. Less is known about the flight heights in bat migration and the attraction toward turbines. Some wildlife agency staff theorize that taller turbines in open areas may attract bats from a greater distance for foraging and swarming and thus greater potential for impacts.  

Beyond direct impacts (collisions), there are other wildlife-related issues to consider with taller turbines:

How can wind developers consider these potential impacts during design? For one, taller turbines are often used for project sites with lower wind regimes. In such situations, it becomes more important to consider potential production limitations from higher cut-in speeds for bat minimization. Results of alternatives analyses will depend on what comparisons are made. Achieving the same power capacity with taller and fewer turbines vs. smaller and more turbines will provide less habitat disturbance and fragmentation as well as a likelihood of overall lower rotor sweep area. Whereas evaluating the same number of turbines but comparing taller turbines with smaller turbines is more likely to have greater collision risk and habitat impacts with the taller turbine alternative.  

Moving forward, study in this area must begin to grab at other factors as well. Studies at modern facilities will be important to determine if the wealth of post-con mortality monitoring data based on older, smaller turbines is still a good fit for modern, taller turbines. Also, to determine which metric is most representative of mortality rates, on a per turbine or per MW basis, or even per RSA? Currently, R&D on bat deterrent systems are being tested on operating turbines smaller than current generation models, so will the lessons learned be applicable to taller models? Project developers may want to study larger search areas and/or adjusted statistics for post-construction mortality monitoring on taller turbine models as well.

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