National Geographic News recently published a story by Victoria Markovitz on the size and relative greeness of wind turbines. In it, she summarizes a new study by the Swiss Federal Institute of Technology in Zurich.
Those researchers did an energy accounting of commercial-scale wind turbines in Europe, taking into account the energy they produce as well as the energy required to build, transport, maintain, and dispose of them. They found that overall, the turbines were getting more favorable in terms of energy balance as time went on, because they were producing more energy while not taking substantially more energy to produce. The researchers attributed this largely to the fact that turbine size went steadily up.
The study noted that over the past 30 years, wind turbines have more than quadrupled in size. “With each doubling of wind-turbine manufacturing over time, the Swiss researchers found, the global warming potential per kilowatt-hour of electricity dropped 14 percent,” writes Markovitz.
Markovitz went on to write that wind turbine experts are now hesitant to quote a maximum upper limit for turbine size, since many have been proven wrong in the past.
Challenges of Bigger Wind Turbines
In the book I recently co-wrote with Kevin Shea, Build Your Own Small Wind Power System, we investigated the relationship between wind turbine size and energy production. As the equation at the bottom of this post shows, when you are dealing with most wind turbines (that is horizontal axis, or “propeller style), the amount of power you can produce is determined by the square of the blade radius. That means increasing the size of the turbine has an exponential effect on power.
However, there are a number of drawbacks to getting a bigger turbine, beyond the ones covered by the Swiss study (a compensated-for amount of energy cost in terms of production, transportation, maintenance, etc.):
- Bigger turbines cost a lot more than small ones. Since they can produce a lot more energy this is often a good economic tradeoff, but you need to be able to afford the thing in the first place. If you have expensive financing you may be fighting a losing battle.
- Bigger turbines have problems with mechanical stress. Large turbines aren’t as new as many think: In 1941 a 1.25 MW wind turbine was installed in Vermont. It lasted only a matter of hours before being doomed by mechanical failure. Wartime shortages meant it couldn’t be fixed. New space-age materials have helped deliver bigger turbines with less weight and less stress, but the engineering challenges are substantial, adding to their cost.
- Bigger turbines have bigger “footprints,” meaning they stand taller and require more land or sea. As Markovitz mentioned, this can mean they are prevented by laws that govern heights of structures, zoning, and so forth. True, laws can be changed, but wind turbines aren’t always popular.
- Large wind turbines have more risk. What happens if climate change means your grassy knoll is no longer windy? Or your computer-guided system fails? Or it falls on someone or catches fire? The bigger the turbine, the more expensive it is to make changes.
- The bigger the turbine, the more likely it is to make noise. In the U.S., there are considerable setback requirements for large turbines, and they are generally a relatively quiet technology, although they make some noise that has bothered some people.
- There currently is not good evidence that big wind turbines are safer for birds and bats. This is commonly claimed by the industry, since the blades turn slower and should therefore be more visible. But they also have a larger footprint. Still, it should be noted that bird and bat kills from turbines are currently a tiny fraction of the total number of kills, well less than one percent, caused by human beings (largely from strikes with buildings and towers, not to mention pollution, domestic cats, and habitat loss).
- Bigger blades may not be as important as higher towers. The most important factor in wind energy production is wind velocity (see below), and in most areas average wind speed increases steadily with height, owing to less turbulence and ground drag. A higher tower means more wind but it is more expensive to build, so designers usually try to optimize them with the biggest turbine that will fit. That’s why “bigger” usually means taller and broader.