Making sense of trends and data

Wind Energy

Of Bats and Blades

Published 6.1.2017
Wind power has enjoyed significant growth globally in the past decade as societies attempt to lower emissions to combat climate change. No energy source is without environmental impact, and wind is no different. Beyond the aesthetic and noise issues with wind installations, wind turbines are known to kill birds and bats. The industry is aware of the wildlife issues, and, in stark contrast with legacy power sources, has adopted mitigation strategies.

The simplest and most effective mitigation strategy to date is to slow or stop the blades spinning at lower wind speeds or during known bat migration periods. Slowing or stopping the blades is easier to accomplish with newer turbines that are remotely controlled. For older turbines, which have to be manually adjusted, this is a more expensive solution.
Fatalities at wind turbines may threaten population viability of a migratory bat
W.F. Frick a,b,, E.F. Baerwald c,d, J.F. Pollock b, R.M.R. Barclay c, J.A. Szymanski e, T.J. Weller f, A.L. Russell g, S.C. Loeb h, R.A. Medellin i, L.P. McGuire j

  • Models the worst case scenario for hoary bat populations over time.
  • Model assumes no mitigation.
  • Notes existing mitigation options can achieve 93% effectiveness.
The effects of wind turbines on wildlife is a hot topic in environmental research. In a February 2017 paper, Frick et al. (reference listed to the left) modeled the potential effects of wind turbines on the population of hoary bats. That bats are killed by turbines is known, and the industry has tried to develop mitigation technology.

As noted above, the best mitigation thus far is to slow or even stop the turning of the turbines during the known migration period. However, very little seems to be known with certainty about the hoary bats and their migration.

Because they do not have actual data about the bats, they asked a group of experts to estimate the population effect using data they have for other species. This work is therefore by definition speculative. They took steps to avoid “group think” but basically, this modeling is based on expert best guesses of the effects.

There have been numerous efforts to quantify the number of turbine related bat deaths. The consensus seems to be that a 2013 estimation is the most accurate.

Arnett and Baerwald (2013) estimated the number of bats killed by wind turbines in Canada and the U.S. in 2012 (range: 196,190–395,886), of which 38% were hoary bats.

This estimated level of carnage is considered more conservative than previous efforts. Using these numbers and the 2014 mega wattage of wind turbine generated power, the authors calculated a death per megawatt benchmark, but they do not list it in the paper.

Looking at deaths/megawatt is a simple way to normalize the data to the amount of wind power generated. However it’s not clear that all sizes of turbine would have the same effect. Using the speculative numbers from their experts, they modeled the rate of population that would be required if their rate of bat death is accurate. Obviously, smaller bat populations are harder hit

The modeling excluded any effects of mitigation. The authors do acknowledge that up to 93% of bat deaths can be avoided with simple mitigation techniques, and that these techniques have been known for years.

The paper suggests that the hoary bat ought to be moved up the list of protected species— which is not going to happen under Trump. The biggest weakness of the speculation is that they didn’t include the known effects of mitigation. The wind industry has adopted voluntary standards designed to mitigate bat and bird death— and to avoid further regulation. This is a notable difference to the way coal, fracking or nuclear industries react to environmental concerns and stems, at least in part, from the industry's own claims of being environmentally friendly.
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Hoary bat
Image source

As noted above, the issue of bats and turbine blades has been studied— for years. Researchers are collecting data and counting corpses, but they are also finding that the bats don’t always fly in the same patterns at a given site. Hoary bats are long-lived and don’t reproduce often, which is one reason the issue has generated such research interest.

Most bat deaths in the US occur in the temperate regions because that's where the bats are commonly found. The best way to avoid bat deaths for new turbine construction is to locate the blades in an area away from the bat flight paths. Researchers have not been able to predict with any accuracy what type of site will result in bat death— beyond that it’s not a good idea to build right by a roost. That said, a viable solution to the issue for all turbines of any location was identified years ago.

Arnett (2005) was the first to employ daily carcass searches and relate them to weather variables, discovering that most bats were killed on low-wind nights when power production appeared insubstantial. Based on this approach, Arnett et al. (2008) estimated that 82–85 % of bat fatalities at two facilities in the eastern USA occurred on nights with median nightly wind speeds of <6 m/s. Since this pivotal discovery, studies worldwide document that most bat fatalities occur during low-wind periods.

Bottom line, the issue is easily mitigated and at (relatively) low cost.

Few studies have disclosed actual power loss and economic costs of operational mitigation, but those that have suggest that <1 % of total annual output would be lost if operational mitigation was employed during high-risk periods for bat fatalities.

Other methods of mitigation have been considered, including radar, GPS tracking, ultrasonic transmitters, and painting the blades different colors (which is an attempt to stop bird deaths). However, the best method remains slowing to stopping the blades in low wind.


How do the number of wildlife deaths due to wind power compare to fossil fuel or nuclear generated power? That question has been studied too.

It estimates that wind farms are responsible for roughly 0.27 avian fatalities per gigawatt-hour (GWh) of electricity while nuclear power plants involve 0.6 fatalities per GWh and fossil-fueled power stations are responsible for about 9.4 fatalities per GWh. Within the uncertainties of the data used, the estimate means that wind farm-related avian fatalities equated to approximately 46,000 birds in the United States in 2009, but nuclear power plants killed about 460,000 and fossil-fueled power plants 24 million.

No source of power is without environmental effects.
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Source: Global Wind Energy Research Council

Wind Past and Future

According to the Global Wind Energy Council (GWEC), 2016 was another good, but not record, year for wind turbine installation, with 54,642 megawatts or wind capacity brought online. For comparison, 2015 was an even better year, with 63,633 megawatts of wind capacity. These are maximum possible turbine capacity, not the actual generated power. Turbines seldom operate at maximum capacity, as power generated is a function of wind speed.

GWEC is a wind industry trade organization with the expected bias towards the industry. Most of their efforts are in developing markets so as not to step on their members efforts.

As an industry, wind power represents only 4% of the total energy generated. Burning fossil fuels, particularly coal is still the most common manner of generating electrical power. Per GWEC's modeling, wind power will represent 6-8% of global power generation in 2020, and 18-20% of generation by 2030. To meet those targets many more wind turbines will need to be built and connected to the grid.

GWEC's modeling assumes that the protocols and emission levels established in the Paris Accord will be realized. In the era of Trump this cannot be assumed to be a given. Trump hemmed and hawed over whether or not the US will remain part of the Agreement, and though he was lobbied to remain from a wide number of people, including his Secretary of State, Rex Tillerson. Tillerson, as well as others in the energy industry, told Trump to stay in the agreement so that the US and its industries keep their "seat at the table."

Trump, however, announced the US will withdraw from the agreement. It is too soon to know what the repercussions will be for this arbitrary decision, nor is it certain that the rest of the world will want to renegotiate the deal. In the short term, the Chinese and Indian governments look to increase their leadership on the world stage. The bottom line for the wind industry is that using the Paris Agreement goal as justification may be less of a selling point in the US market.

China has led the world for the past few years in terms of installation, though the Chinese continue to lag the US when it comes to generation. Part of the issue in China is that the region with the greatest wind assets is far from the industrial regions where power demand is surging. A second issue is the lack of infrastructure to transmit the the power.

India's government has said it would like to lead in renewable energy, but it is solar, not wind that has grown rapidly. GWEC predictions suggest wind may soon be growing on par with solar. India has several regions with decent wind assets, but solar is likely to dominate the country's renewable sector.

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