With the current level of Knowledge and technology, wind developers have two options to reduce the risk of harming bats at wind farms. First, developers can site projects in areas with the lowest bat populations possible. Second, they can curtail the operation of turbines in such a way as to reduce the risk of bat deaths. Unfortunately, both siting and curtailment are costly for wind developers to implement in addition to being limited in their ability to protect bats.
The main problem with the siting solution is that while it is possible to avoid building turbines near large bat caves, bats migrate many miles from their caves to roost and feed. Therefore, even if a developer avoids bat caves, the site it chooses may still prove attractive to feeding, breeding and roosting bats. Similarly, turbine curtailment essentially takes turbines offline at certain times, thereby representing a loss of revenue over the life of a wind project.
Curtailment comes in two varieties. The first is seasonal curtailment, where turbines run only in the winter hibernation season. The second is daily curtailment, where the cut-in of turbines is delayed in the morning and at night to allow bats to vacate the area. Both forms of curtailment – but especially seasonal curtailment – represent functional liens on a project’s revenue stream.
Regarding seasonal curtailment, for example, Invenergy has been told by a federal judge that it cannot operate the turbines at its Beech Ridge project in West Virginia, from approximately April through November. This time period corresponds to the months when bats are active, but the curtailment reduces the return on investment from the project by almost half.
Daily curtailment of wind turbine operation has recently been found to lower bat mortality at wind facilities. At night, the cut-in speeds are increased when winds are stronger. The rationale is that by increasing the turbine’s cut-in speeds, the turbines commence operating at later hours, therefore allowing bats to vacate the area. Three studies worldwide have tested whether increasing the minimum turbine cut-in speed reduces bat fatalities. These studies demonstrated that bat fatalities were reduced by 50% to 87%.
However, curtailment reduces output over the life of a project, and wind companies are loath to adopt it. In addition, both types of curtailment are still just ex post facto methods, in that they do nothing to prevent bats from entering wind farms in the first place.
—from North American Windpower, May 2012