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Flying windmills tapping jet stream wind currents may sound far fetched, but groups in the US, Netherlands and Canada say such devices may soon be within reach. If successfully developed, they could harness an enormous amount of reliable, renewable energy.

鈥淚f we were able to tap 1% of the wind energy at high altitude, that would be enough to supply all the world鈥檚 energy needs,鈥� says David Shepard, president of renewable energy startup Sky WindPower, in Coronado, California.

Shepard says he plans to begin building a 220kw Flying Electric Generator (FEG) prototype later in 2007, and hopes to have it flying at 4500 metres by the summer of 2009. Sketches of the contraption show four rotary blades connected to an H-shaped aluminum frame, which is tethered to the ground by a high-voltage power line.

The current prototype (see video, right) is based on an earlier model that was first flown in the 1980s by Sky WindPower鈥檚 chief engineer, Bryan Roberts of the University of Technology in Sydney, Australia (See Reach for the sky).

Both models take off like helicopters powered by their electric tether. Then, once their rotors catch the jet-stream wind, they feed electricity back to the ground like traditional wind turbines with their rotors rotated to the horizontal.

Wing-shaped kites

Roberts, Shepard and others recently wrote a that sets out the details of their idea.

Shepard expresses great confidence in his projections, saying clusters of the devices will soon be flying at heights up to 9000 metres, providing renewable energy that out-competes all other fuels. 鈥淚t鈥檚 going to beat fossil fuels and nuclear power economically within 10 years,鈥� he claims.

Researchers at Delft University of Technology in the Netherlands are working on a slightly different high-flying generator called a . The mill is made up of a series of wing-shaped kites that generate energy by rotating a large loop to which they are connected.

Curved fins

But a third group pursuing airborne wind generation says high-altitude jet stream currents are not worth the trouble. 鈥淲hen you go that high, there is this massive cable that you have to deal with,鈥� says Mac Brown, CEO of renewable energy start-up , of the electric tether connecting the generator to the ground.

鈥淲e鈥檝e looked at all kinds of wind maps, and there is loads of wind in the 180 to 300 metre range,鈥� he says.

Brown鈥檚 company uses a helium-filled blimp with curved fins that harness this lower altitude wind (see video, right). This causes the aircraft鈥檚 midsection to spin, rotating power generators at each end.

The company flew a 3 x 7 metre prototype in 2006 that was too small to carry the weight of these generators. Brown says he now hopes to build a larger balloon 鈥渢he size of a three-story house鈥� and capable of lifting 1100 kg.