Today National Geographic News 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.

. 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.

. 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.

,” 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.

. 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.

. 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).

. 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.

View Images In 1941 a big wind turbine rated at 1.25MW was installed in Vermont, but it soon failed due to mechanical stress. Photo: U.S. DOE

Diverse Future for Wind Power

For Build Your Own Small Wind Power System I interviewed Morten Albaek, senior vice president of global marketing for Vestas, the world’s largest wind turbine maker. He told me that his company’s strategy is currently to keep working on bigger and better turbine designs. Still, he said he thinks there is room to develop alternative designs, such as micro-turbines and possibly even new types of generators.

Companies like Makani, Magenn, and KiteGen have been researching kite-lofted wind turbines that get even higher, into stronger winds than the tallest towers. They are at early testing phases, but already entrepreneurs have been pitching fleets of high turbines lofted by boats or anchored to the sea floor or desert bedrock.

Early research by John Dabiri at Caltech has suggested that densely packing counter-rotating vertical-axis wind turbines might enable them to extract even more energy out of a given area of wind than today’s big commercial horizontal turbines. That has yet to be proven in the field at any scale, and many wind industry experts remain skeptical about vertical axis designs, but who knows what engineers will uncover in the coming decades?

Others have suggested that the most efficient landscape might be small turbines interspersed with large ones, to harvest as much energy out of the wind as possible.

Albaek told me, “We think most wind energy will [continue] to be developed by classic large wind farms, but we think the next developments in wind energy technology will be in micro-turbines.” The amount of energy we will be able to get from micro-turbines is going to be limited, but important, compared to what we will get from large classical wind turbines, but I really hope the micro-turbine industry sees success.”

For the near future, expect to see more turbines like the 10MW SeaTitan and beyond.

Wind Energy Equation

For those who want a detailed look, the energy produced by a wind turbine can be estimated by this equation, often called the wind energy equation. It shows that increasing the swept area will always give you more power, and since the area swept is usually a circle (at least for horizontal axis wind turbines), it has a squared relationship to blade length:

Power output in kilowatts = k x Cp x 1/2 x p x A x Vcubed

k is the constant 0.000133 to yield power in kilowatts; Cp is the power coefficient of the turbine, never more than 0.59 and typically 0.25 to 0.45; p is the air density at your site in pounds per cubic foot; A is the swept area of your rotor (usually you use pi x radius squared for horizontal axis turbines); V is the wind speed (by far the most important variable because of the cube)

But bigger blades have to be weighed against your budget, site contraints, and the other factors… The most important consideration is to get your turbine into strong winds.

Read the story on wind turbine size >>