An airborne wind turbine is a design concept for a wind turbine that is supported in the air without a tower.[1] Airborne wind turbines may operate in low or high altitudes; they are part of a wider class of airborne wind energy systems (AWE) addressed by high-altitude wind power. When the generator is on the ground,[2] 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. Airborne turbine systems would have the advantage of tapping an almost constant wind, without requirements for slip rings or yaw mechanism, and without the expense of tower construction. Kites and 'helicopters' come down when there is insufficient wind; kytoons and blimps resolve the matter. 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, an aircraft exclusion zone. As of 2011, no commercial airborne wind turbines are in regular operation.[3]
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An aerodynamic airborne wind power system relies on the wind for support.
Bryan Roberts, a professor of engineering at the University of Technology, in Sydney, Australia, has proposed a helicopter-like craft which flies to 15,000 feet (4,600 m) altitude and stays there, held aloft by wings that generate lift from the wind, and held in place by a cable to a ground anchor. According to its designers, while some of the energy in the wind would be 'lost' on lift, the constant and potent winds would allow it to generate constant electricity. Since the winds usually blow horizontally, the turbines would be at an angle from the horizontal, catching winds while still generating lift. Deployment could be done by feeding electricity to the turbines, which would turn them into electric motors, lifting the structure into the sky.
The Dutch ex-astronaut and physicist Wubbo Ockels, working with the Delft University of Technology in the Netherlands, has designed and demonstrated [4] an airborne wind turbine he calls a "Laddermill". It consists of an endless loop of kites. The kites lift one end of the endless loop, (the "ladder") up, and the released energy is used to drive an electric generator.
A Sept'09 paper[5] from Carbon Tracking Ltd., Ireland has shown the capacity factor of a kite using ground-based generation to be 52.2%, which compares favorably with terrestrial wind-farm capacity factors of 30%.
A team from Worcester Polytechnic Institute in the United States has developed a smaller-scale [6] with an estimated output of about 1 kW. It uses a kiteboarding kite to induce a rocking motion in a pivoting beam.
The Kitegen uses a prototype vertical-axis wind turbine. It is an innovative plan (still in the construction phase) that consists of one wind farm with a vertical spin axis, and employs kites to exploit high-altitude winds. The Kite Wind Generator (KWG) or Kitegen is claimed to eliminate all the static and dynamic problems that prevent the increase of the power (in terms of dimensions) obtainable from the traditional horizontal-axis wind turbine generators. Generating equipment would remain on the ground, and only the airfoils are supported by the wind. Such a wind power plant would be capable of producing the energy equivalent to a nuclear plant, while using an area of few square kilometres, without occupying it exclusively. (The majority of this area can still be used for agriculture, or navigation in the case of an offshore installation.)
Dave Santos, of KiteLab in Ilwaco, Washington, has been advancing single-surface wingmills to generate useful electricity with a ground-based generator.[7]
The Rotokite [8] is developed from Gianni Vergnano's idea. It uses aerodynamic profiles similar to kites that have been rotated on their own axis, emulating the performance of a propeller. The use of the rotation principle simplifies the problem of checking the flight of the kites and eliminates the difficulties due to the lengths of cables, enabling the production of wind energy at low cost. The Heli Wind Power is a project of Gianni Vergnano that uses a tethered kite.
In August 2011 the German company SkySails, producer of kites for ship propulsion, announced a kite-based wind power system for on- and offshore applications that is supposed to be "30% cheaper than current offshore solutions".[9]
An aerostat-type wind power system relies at least in part on buoyancy to support the wind-collecting elements. Aerostats vary in their designs and resulting lift-over-drag aerodynamic characteristic; the kiting effect of higher lift-over-drag shapes for the aerostat can effectively keep an airborne turbine aloft; a variety of such kiting balloons were made famous in the kytoon by Domina C. Jalbert.
Balloons can be added to the mix to keep systems up without wind, but balloons leak slowly and have to be resupplied with lifting gas, possibly patched as well. Very large, sun heated balloons may solve the helium or hydrogen leakage problems.
An Ontario based company called Magenn [10] is developing a turbine called the Magenn Air Rotor System (MARS). The 1,000-foot (300 m)-wide MARS system uses a horizontal rotor in a helium suspended apparatus which is tethered to a transformer on the ground. Magenn claims that their technology provides high torque, low starting speeds, and superior overall efficiency thanks to its ability to deploy higher in comparison to non-aerial solutions.[11] The first prototypes were built by TCOM in April 2008.No production units have been delivered. [12]
The Twind Technology concept uses a pair of captive balloons at an altitude of 800 meters. The tether cables transmit force to a rotating platform on the ground. Each balloon has a sail connected to it. The two balloons move alternately, the balloon with the sail open moves downwind and draws the other balloon upwind, and then the motion reverses. The tether cable can be used to turn the shaft of a generator to produce electrical energy or perform other works (grinding, sawing, pumping).
Sky Windpower estimates that its technology will be capable of producing electricity for $0.02 per KWh, while a system of raising a kite to a high altitude while turning a generator on the ground, and then changing its shape so that it can be drawn back down with less energy than it produced on the way up, has been estimated to be capable of producing electricity for $0.01 per KWh[13] - both numbers being significantly lower than the current price of non-subsidized electricity.
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