Airborne wind turbine
 Altaeros Prototype 2012
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An airborne wind turbine is a design concept for a wind turbine with a rotor supported in the air without a tower, thus benefiting from more mechanical and aerodynamic options, the higher velocity and persistence of wind at high altitudes, while avoiding the expense of tower construction, or the need for slip rings or yaw mechanism. An electrical generator may be on the ground or airborne. Challenges include safely suspending and maintaining turbines hundreds of meters off the ground in high winds and storms, transferring the harvested and/or generated power back to earth, and interference with aviation.
Airborne wind turbines may operate in low or high altitudes; they are part of a wider class of Airborne Wind Energy Systems (AWES) addressed by high-altitude wind power and crosswind kite power. When the generator is on the ground, then the tethered aircraft need not carry the generator mass or have a conductive tether. When the generator is aloft, then a conductive tether would be used to transmit energy to the ground or used aloft or beamed to receivers using microwave or laser. Kites and 'helicopters' come down when there is insufficient wind; kytoons and blimps may resolve the matter with other disadvantages. Also, bad weather such as lightning or thunderstorms, could temporarily suspend use of the machines, probably requiring them to be brought back down to the ground and covered. Some schemes require a long power cable and, if the turbine is high enough, a prohibited airspace zone. As of July 2015, no commercial airborne wind turbines are in regular operation.
An aerodynamic airborne wind power system relies on the wind for support.
Miles L. Loyd proposed and analyzed an efficient AWES in his work "Crosswind Kite Power" in 1980. Power output of AWES with crosswind wing motion is proportional to a square of a lift/drag ratio of the wing. Such AWES is based on the same aerodynamic principles as a conventional wind turbine (AWES), but it is more efficient because the air speed is constant along the wing span and the aerodynamic forces are resisted by tension of a tether, rather than by bending of a tower.
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