Do all modern windmills look alike?

Answers to your questions about the news.
April 28 2010 6:46 PM

Why Are Windmills Always White?

And why do they always have three blades?

Windmills.
Windmills

The federal government has green-lighted the nation's first offshore wind farm, to be built off the coast of Cape Cod, Mass. Opponents claim that 130 white, three-bladed turbines will detract from the natural beauty of Nantucket Sound. Why do all modern windmills look the same?

So they're unobtrusive. A windmill's noise is directly proportional to the speed of its rotor tips. Two-bladed turbines have to spin faster than their three-bladed competitors to generate the same amount of energy. As a result, the whooshing sound they emit is somewhat louder. Two-bladed windmills would be a sensible choice for a remote, offshore wind farm like the one in Cape Cod, since they're just as efficient as the three-bladed models and cheaper to produce. But manufacturers—who cater to the densely populated and wind-power-oriented countries of Europe—have switched almost exclusively to producing the latter.

Advertisement

The placement of rotors relative to the tower is also a design controversy with acoustics implications. The downwind design, with the tower between the blades and the wind, is more structurally sound. (Think of a pinwheel: If you don't grip it tightly, it will tend to rotate into the downwind position.) The problem is that the tower creates a dead spot for airflow, which stresses the spinning rotors and generates a repetitive whop that can carry for miles. Right now, most manufacturers favor the upwind layout.

The white paint, which many localities require by ordinance, is also a matter of aesthetics. City planners seem to think white windmills are less of an eyesore. The white also reflects sunlight, which minimizes expansion and cracking of the gel coat that protects the fiberglass composite rotors. Not all windmills are white, though. Some Midwestern turbines are yellow to match the grain. (This doesn't work so well in the spring, when the crops are green.) German windmills are sometimes painted dark green at the bottom to blend into the forest. European rotors usually have a red stripe to make them visible to aircraft. Engineers once tried painting the rotors black to absorb sunlight and prevent icing, but it didn't seem to help much.

Hollow, tubular towers have vanquished the old girder design, because they discourage birds from landing on them. (Birds and windmills don't mix.) The tubes are also favored by construction crews, who can climb up a tower to repair it from the inside, protected from the elements.

The biggest design question for most engineers is rotor length. The energy a windmill generates is proportional to the area of the rotors' circular sweep, so energy increases proportionally to the square of the blade length. However, the volume of the rotor, which determines the cost, is proportional to the cube of the length, and increases faster than energy production. As we get better at materials engineering, the rotors will get longer. But at any given time the arms of a windmill will be built out to the length that maximizes energy return relative to the cost of production. The rotors on modern windmills are sometimes as much as 200 feet long. As such, transportation can also be a problem.

Got a question about today's news? Ask the Explainer.

Explainer thanks Douglas E. Adams of the Purdue Energy Center, Scott Larwood of the University of the Pacific, James Manwell of the University of Massachusetts Renewable Energy Research Laboratory, and Jonathan Naughton of the University of Wyoming Wind Energy Research Center.

Become a fan of Slate and the Explainer on Facebook. Follow us on Twitter.

TODAY IN SLATE

Frame Game

Hard Knocks

I was hit by a teacher in an East Texas public school. It taught me nothing.

Chief Justice John Roberts Says $1,000 Can’t Buy Influence in Congress. Looks Like He’s Wrong.

After This Merger, One Company Could Control One-Third of the Planet's Beer Sales

Hidden Messages in Corporate Logos

If You’re Outraged by the NFL, Follow This Satirical Blowhard on Twitter

Sports Nut

Giving Up on Goodell

How the NFL lost the trust of its most loyal reporters.

How Can We Investigate Potential Dangers of Fracking Without Being Alarmist?

My Year as an Abortion Doula       

  News & Politics
Weigel
Sept. 15 2014 8:56 PM The Benghazi Whistleblower Who Might Have Revealed a Massive Scandal on his Poetry Blog
  Business
Moneybox
Sept. 15 2014 7:27 PM Could IUDs Be the Next Great Weapon in the Battle Against Poverty?
  Life
Dear Prudence
Sept. 16 2014 6:00 AM Can of Worms Prudie offers advice to a letter writer who wants to blackmail a famous ex with tapes of his fetish.
  Double X
The XX Factor
Sept. 15 2014 3:31 PM My Year As an Abortion Doula
  Slate Plus
Tv Club
Sept. 15 2014 11:38 AM The Slate Doctor Who Podcast: Episode 4  A spoiler-filled discussion of "Listen."
  Arts
Brow Beat
Sept. 15 2014 8:58 PM Lorde Does an Excellent Cover of Kanye West’s “Flashing Lights”
  Technology
Future Tense
Sept. 16 2014 7:36 AM The Inspiration Drought Why our science fiction needs new dreams.
  Health & Science
Bad Astronomy
Sept. 16 2014 7:30 AM A Galaxy of Tatooines
  Sports
Sports Nut
Sept. 15 2014 9:05 PM Giving Up on Goodell How the NFL lost the trust of its most loyal reporters.