Through the years there have been many designs aimed at harvesting high-altitude winds. The attraction of these schemes is that winds aloft blow harder and are more consistent than those near the ground. High-altitude wind has between 2 and 30 times more power density than surface wind.
The latest design in this camp comes from Joby Energy in Santa Cruz, Calif., which has a design resembling a multi-wing kite. Once aloft, propellers driving motor/generators harvest tropospheric winds. The same rotors raise the kite into the air up off the ground
A computer differentially adjusts rotor speeds to keep the device in the air flying in a circular path at the end of a tether, much like a kite in the wind. The super-high winds you find in the troposphere spin the turbines rapidly enough that there is no need for gearboxes. The electricity generated by the turbines is transmitted to the ground via a reinforced composite tether that anchors the kite to the ground.
One problem: The U.S. Federal Aviation Administration has restricted the altitude initially to below 600 m. Joby is working on a test system for that altitude able to generate 30 kW. Plans are to then scale up to a 100 kW prototype and then to an initial set of turbine arrays generating 300 kW.
If the tether breaks the kite would have enough smarts on-board to land itself. Similarly, it would also gracefully come to earth in the event of gale-force winds or periods of no wind at all. And if a few of the turbines die, the kite can still remain aloft.
If everything goes well, Joby hopes to begin manufacturing its kites in 2012.
More info: http://www.jobyenergy.com/