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In the short history of wind development, the industry has come a long way in better understanding how to protect wildlife and minimize impacts. But in setting higher thresholds and limitations on turbine activity, regulators have tended to take a one-size-fits-all approach based on limited data.

Wildlife-related assessments of potential impacts from wind projects typically focus on habitat: topography, forest type and proximity to habitat features.

However, mortality patterns are remarkably similar among projects in a wide range of habitat and landscape types.

The ever-changing characteristics of air in the rotor zone and its suitability as habitat for foraging or migration are largely ignored but may have a greater influence on risk of turbine-related mortality than do traditional habitat features. Indeed, the risk of bat mortality changes constantly based on a number of conditions, not just seasonal migration patterns. In order to better understand the influence those changes have on bat activity, the wind industry needs to treat the aerosphere – the relatively thin layer of atmosphere that supports life – as a habitat.


Curtailment standards

It is understood that the risk of turbine-related bat mortality disappears when turbines are fully feathered to prevent rotation. A conservative general approach to curtailment has been to set a high threshold for increased cut-in speed – right now, it is 6.9 m/s from April through October – with the expectation that this level will minimize bat mortality in most settings and conditions. Although that standard does work, it is expensive, not always efficient and possibly no more effective than alternative approaches.

In other words, many wind farms are trying to operate at that standard during migratory periods or on nights too cold for bat activity – when they do not need to. A multitude of alternative scenarios may prove similarly effective without the major costs that this level of curtailment incurs. The problem is few, if any, recent studies have adequately compared these alternatives.

The wind industry needs more research and data on other approaches to curtailment. This task should not fall entirely on the agencies. Developers could be leading the charge to commission studies of alternative methods of keeping bat mortality low during operation. On one hand, this data could justify the 6.9 m/s curtailment threshold. On the other hand, it could open up options that help wind farms operate more efficiently while mainlining the same high level of protection for bats.

One place to start that thinking is to consider that the suitability of the aerosphere for bats changes constantly based on physical flight characteristics and prey availability, likely due to a large number of factors. The environmental assessment process tends to consider the conditions of all other types of habitat in siting a wind farm, but it often doesn’t define the aerospheric conditions as habitat markers in the same way. Similarly, operational models focus curtailment strategies on migration patterns, which are typically seasonal. But air conditions are constantly changing – even in the course of a night or day – and can affect how and when bats move through the atmosphere in or outside of a given migration phase.

For example, temperature is a key factor in bat behavior – if it is cold, fewer bats are flying. Similarly, if it is raining or snowing, the bats tend to stay put. A number of other factors have these same kinds of effects, from fog, cloud cover and air mass, to wind speeds, dew points and humidity. Wildlife scientists can already make some educated assumptions about curtailment in these conditions, but with more research, they will be able to predict them. With that data and the forward-looking predictions it allows, wind farm operators will be able to better tailor curtailment strategies to focus on periods and conditions when risk is highest (i.e., when the rotor zone air space habitat is most suitable for bats).

Today’s sophisticated turbine models can incorporate these factors and their fluctuations based on what’s happening in the air. As more data is collected, small adjustments can be made to the models that will continue to reduce fatality rates.

Details such as the time intervals over which wind speed and other variables are calculated, as well as which turbines are taken on and off curtailment, likely affect the efficiency and effectiveness of curtailment.




What’s next?

Simply put, there just isn’t much information out there about how the aerosphere works and functions as habitat. Similarly, wind farm operators and turbine manufacturers have only recently begun implementing curtailment programs, and there is a lot of room to improve the mechanics and design of curtailment systems. With more research on both fronts – whether it comes from academia, the wind industry, regulatory agencies or third-party consultants – all of us will be able to make more sense of how this plethora of information fits together.

Everyone wants to keep the impacts to wildlife as low as possible, but doing so in the most efficient manner creates even more incentive for developers and others to comply and possibly go beyond the minimum.

If the wind industry can create an environment in which developers are able and encouraged to research new, more efficient curtailment methods that find similar or better results, there’s no reason those approaches can’t become acceptable alternatives to the regulatory agencies. And more options open more possibilities for responsible development, which benefits developers, regulators and affected species.


Trevor Peterson and Sarah Boucher are wildlife biologists with Maine-based consultancy Stantec. They can be reached at (207) 729-1199 or at trevor.peterson@stantec.com and sarah.boucher@stantec.com, respectively.

Industry At Large: Environmental Impacts

Taking A New Look At Curtailment Standards

By Trevor Peterson & Sarah Boucher

Subtle differences in turbine operation could lead to significant changes in avian fatality rates.





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