A Worldchanging post rounds up three different airborne power-generation systems -- a flying windmill, a windmill-equipped zeppelin, and a kite-based windmill.
According to their figures, one flying windmill rated at 240kW with rotor diameters of 35 feet could generate power for less than two cents per kilowatt hour--that would make them the cheapest power source in the world. For greater power needs, several units would be operated in the same location--Sky Windpower says that an installation "rated at 2.81 megawatts flying at a typical U.S. site with an eighty percent capacity factor projects a life cycle cost per kilowatt hour at 1.4 cents." And they would have far better uptime than most windmills--since the jetstream never quits, they should operate at peak capacity 70-90% of the time. Output would also be less dependent on location than it is on the ground, simply because terrain doesn't matter much when you're at 35,000ft; however, since the jetstream and other "geostrophic" winds don't blow much at latitudes near the equator, it would be useful primarily for middle- and higher-latitudes.Link