The study, published in late August, examined 188 bats killed at turbines the previous night and found 87 had no external injury that would have been fatal. Of 75 fresh corpses on which autopsies were conducted in the field, 32 had obvious external injuries, but 69 suffered internal haemorrhaging consistent with a condition known as barotrauma, which is tissue damage caused by rapid or excessive drop in air pressure that, in modern turbines, occurs at the blade tip vortices and is in the range of five to ten kilopascals. Only six bats had an external injury but no internal tissue damage.
"Because bats can detect objects with echolocation, they seldom collide with man-made structures," says project leader Erin Baerwald, a PhD candidate at Calgary's university. "An atmospheric pressure drop at wind turbine blades is an undetectable -- and potentially unforeseeable -- hazard for bats, thus partially explaining the large number of bat fatalities at these specific structures."
Bats are more susceptible to barotrauma than birds because of differences in their respiratory systems, the researchers note. Bats have large, pliable lungs that can overexpand when exposed to sudden pressure drops, bursting surrounding capillaries. Bird lungs are more rigid and tube-like and better able to withstand air pressure changes.
While Baerwald's research helps solve the mystery of why large numbers of migratory bats are killed at wind farms, it is some of the other work that her team and the project owners are doing at Summerview that TransAlta Wind's Jason Edworthy expects could ultimately point to a solution. "In terms of where our research is going, I don't think these results make a lot of difference at this point. The bats are still getting too close to the blades," he says. "We still don't know why they seem to be attracted to the turbines, and we think they are attracted to the turbines. The implications of this are that they may be taking evasive action at the very last minute, but then boom, they fall into this ten kilopascal pressure drop."
TransAlta brought the Summerview project online in autumn 2004 and began to see large numbers of bat fatalities during the fall migration the following year. The University of Calgary study was launched in 2006 and last year the focus of the research shifted to the question of what can be done to reduce the number of deaths.
Building on observations at Summerview and elsewhere that the numbers of flying bats decrease as wind speeds increase, researchers adjusted the cut-in speed of 19 of the 38 Vestas 1.8 turbines at the site to 5.5 m/s between August 1 and September 4. The remaining turbines were allowed to operate with their usual cut-in speed of about 4 m/s.
The results showed a significant reduction in mortality, with an average of 3.9 bat fatalities per turbine at the experimental units, compared to an average of 8.1 deaths at the control turbines.
There are now similar experiments going on at other wind projects across North America to verify the results, says Edworthy. Meantime, TransAlta is upping the ante a bit this year by moving the cut-in speed of experimental turbines at Summerview even higher. "Our hypothesis is that this year, by moving up to a 6 m/s cut-in wind speed, we can reduce fatalities by closer to 90%. The reason is that other research that just monitors bats in different wind speed shows most species of bats have a 90% reduction in animals flying at 6 m/s and above," Edworthy explains.
A reduction in bat mortality of that magnitude would bring Summerview in line with other TransAlta projects where bat deaths have not been an issue. "Those have always been considered acceptable levels, so I think 90% is a pretty good target," says Edworthy.
Raising the cut-in speed, however, does have an impact on project revenue. Electricity from Summerview is sold into Alberta's spot electricity market, so the financial impact depends on what power pool prices and the winds are doing at any given time. But the company estimates there was C$50,000 in lost production at the experimental turbines last year.
TransAlta is looking at ways to use what it has discovered about bat behaviour to strategically operate the turbines during the fall migration period and minimise those losses, says Edworthy. For example, he says, it may only be necessary to change the turbine operating parameters for a few hours a day, from about a half hour before dusk to around 03:00, when migratory activity appears to taper off.
The company's goal is reduce revenue losses by 90% as well. "There is pretty good evidence we should be able to do that through all this strategic stuff," says Edworthy.