Some questions about wind turbines, prairie birds, and small planes

A 15-year-old student in Oklahoma wrote to the North American Platform Against Windpower with questions about the negative aspects of wind turbines for a Future Farmers of America project:

I was wondering, how are these wind turbines affecting the Lesser Prairie-Chickens? I hear they are going extinct! What is the efficiency of windmills? Are they really worth it? Are crop dusters still able to spray fields? And lastly, are there any other animals thought to be affected by these turbines? Thanks for your time.

NA-PAW’s answers:

Hi, ——

These are good questions.

1.  The mating and nesting behavior of Lesser Prairie Chickens (and other ground birds, such as the Sage Grouse) is known to be adversely affected by tall structures, which obviously include industrial-scale wind turbines. The birds abandon breeding areas near tall structures, and the disturbance and fragmentation of their habitat further makes it difficult for them to establish viable populations. Wind Watch has a letter from 5 years ago from the Oklahoma chapter of The Wildlife Society expressing their concerns: https://www.wind-watch.org/documents/letter-to-oklahoma-dept-of-wildlife-conservation/. You might want to contact them about this: http://wildlife.org/oklahoma/node/4.

The U.S. Fish and Wildlife Service is considering listing the lesser prairie chicken as a "threatened" species. Wind Watch has posted a number of news stories on the issue: https://www.wind-watch.org/searchnww.php?q=prairie+chicken&seltab=news.

2 & 3.  "Efficiency" has different meanings in engineering and popular usage. In the engineering sense, the theoretical limit for converting the kinetic energy of wind into electric energy is 59.3%. (For comparison, when burning things for electricity, only about 33% of the fuel's energy can be converted to electricity.) A modern wind turbine can achieve an efficiency up to about 50%. However, it is that efficient at only a narrow range (5-10 mph) of wind speeds midway between the cut-in speed (when the turbine starts to spin) and the "rated" speed (when the turbine generates at is full, or nameplate, capacity, e.g., a 2-MW turbine generating at 2 MW). At its rated wind speed, which typically between 25 and 35 mph, the turbine's efficiency is only 25-30%. As the wind speed increases, the efficiency decreases further as the wind turbine feathers its blades or even brakes them to prevent spinning too fast. Above about 55 mph, the turbine shuts down for safety reasons.

So in engineering terms, a wind turbine is maximally efficient (about 50%) in a narrow (5-10 mph) band of wind speeds halfway between the cut-in speed and the rated speed, with the rate of electricity generation increasing in relation the wind speed. At the cut-in and rated speeds, the efficiency is around 25-30%. Above the rated speed, the efficiency falls off dramatically. Below the cut-in and above the cut-off (around 55 mph), the efficiency is 0, or, more accurately, worse as the turbine uses energy from the grid for its maintenance functions.

Over a period of time, usually a year, the actual production is averaged out to what is called the "capacity factor", which, is necessarily much less than the the nameplate capacity, since that is attained only in wind speeds from about 30 to 55 mph. For examples of capacity factors, see https://www.wind-watch.org/documents/u-s-wind-capacity-factors-2012-preliminary/.

The capacity factor is what most non-engineers refer to (inaccurately) as the wind turbine's efficiency. Many people also erroneously cite the capacity factor as a function of time, eg, saying that a wind turbine or facility with a 30% capacity factor "works" only 30% of the time. In fact, a typical wind turbine generates electricity about two-thirds of the time, though rarely at its rated capacity, and the capacity factor is its overall average output over a complete period of time (usually a year).

For comparison, nuclear plants operate in the opposite way from wind. Nuclear plants operate continuously at full capacity, except when shut down for maintenance or refueling (or accident). Their capacity factors are typically 85-90%, which, unlike for wind, does mean that they are operating 85-90% of the time, because when they are on they are always generating at full capacity. In contrast, wind turbines typically operate about two-thirds of the time, but because their output varies, depending on the wind, their capacity factors may be only around 30%.

Thus wind energy answers to the wind rather than actual consumer demand. In other words, besides requiring a huge (environmentally damaging) infrastructure to collect a meaningful amount of such a diffuse energy source as the wind, it is only by chance that it provides power when the grid actually needs it. This means that that the grid must not only be prepared to operate as if the wind turbines weren't there, but also have extra (conventional power) backup to protect against any sudden drops in the wind. And wind turbines are consumers of electricity when the wind isn't blowing within their optimal range of speed. All of these factors largely undercut the theoretical benefits of wind energy, which is why, after decades, it still provides well less than 1% of the world's electricity.

4.  Such large structures as industrial-scale wind turbines, with turning blades 120-150 feet long, present an obvious flight hazard. Besides the physical obstruction, the turning blades create vortices of turbulence behind them (which is why the turbines themselves are placed so far from each other). This has raised concerns for not only crop service planes, but also for fire fighting and medical rescue.

5.  In the plains, another prominent concern is for the Whooping Crane, whose migration paths cross many areas targeted for wind turbines. The Wildlife Society raised concerns about the whooping crane and the Mexican free-tailed bat in the letter referred to above. The whooping crane was almost extinct 70 years ago, and there are now just 2 wild populations. It remains listed as an endangered species. For news stories, see https://www.wind-watch.org/searchnww.php?q=whooping+crane&seltab=news.

I hope this information is helpful, and feel free to follow up if you like.

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