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Windmills make a lot of sense, giving us free energy that's all around us. But what happens when it's not windy? Build the windmills up high, very high—we're talking between 15,000 and 35,000 feet high, where powerful jet stream winds blow pretty much all the time. Worldchanging describes how to accomplish this with three different approaches to a ground-tethered windmill: a graceful-looking flying windmill with 35-foot rotors (pictured above), a blimp, and a kite. The result? These could be the most cost-effective power generators on or off the planet, cranking out the kilowatts at pennies per. The problems of the things falling out of the sky, acting as highly efficient lighting rods, and being run into by airplanes haven't quite been solved as of yet.

Flying windmills — power from the sky [BoingBoing]