As any kite-flier knows, getting a kite off the ground can involve some effort. But once aloft, a kite can fly seemingly forever. That’s because winds even a hundred feet or so above the ground are stronger and steadier than those close to the surface.
In the jet streams, which flow about six miles above the Earth, winds often exceed 100 mph. Those powerful air currents contain about 100 times as much energy as the world now uses, according to experts.
Even at 600 or 700 feet above-ground, the wind can move twice as fast as at 150 or 200 feet, and it contain eight times as much physical energy.
Modern wind turbines, however, are typically mounted on towers 200 to 300 feet tall. As a result, those icons of renewable energy stand idle for the equivalent of months every year for lack of sufficient wind.
Kite-inspired aircraft could change the equation. The concept behind what’s known as airborne wind energy is to liberate the power-generating component of a wind-energy system — today, the turbine — from the Earth-bound structure that anchors it to the power grid, allowing it to move higher and take advantage of the greater wind velocity up there.
One option that entrepreneurs are exploring is to deploy a kite or gliderlike aircraft that, as it rises, pulls on a cable attached to a generator on the ground. The generator can convert the cable’s unwinding motion into electricity. And when the aircraft reaches the end of its rope, it can glide almost effortlessly back toward the ground, then repeat.
Another approach is to use a wind-powered, unmanned aircraft that carries a turbine-and-generator unit onboard. As the airborne turbine catching the high-altitude breeze and spins, the electricity it generates gets routed down a sturdy electrical wire that tethers the vehicle to a ground station.
Both approaches have been shown to work in principle. Half a dozen or more companies have flown self-piloting prototypes and are working to build bigger, better versions for commercial use.
The biggest challenge, the entrepreneurs say, is perfecting the flight software needed to keep the vehicles flying safely day and night, and to take off and land when necessary. An industrial-scale airborne wind energy plant would need to have many craft in the air, and having human pilots at the controls of each one isn’t practical. So the unmanned aircraft would need to fly expertly enough to stay out of one another’s way, out of airspace used by commercial and military aircraft, and safely off the ground.
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