One should distinguish between true airborne wind energy systems (AWES) and conventional wind turbines, which are raised by buoyancy (rather than a tower). Further, among AWES, systems using a crosswind motion of an airfoil are significantly more efficient than those that do not (i.e., those that use downwind motion of a parachute), so we include only systems that do in our definition of AWES. In AWES, a tethered wing moves crosswind many times faster than the wind, and harvests wind energy from the area, many times exceeding the wing plan area.
Miles Loyd introduced AWES in 1980, in his seminal work Crosswind Kite Power, using hints from a 1976 patent by Payne and McCutchen. Loyd’s ideas were well ahead of the time. Since then, sensors and computational resources, which are required to control airborne wind power systems, became affordable. At the same time, significant progress was made in developing materials for tethers and wings and in wing construction techniques. Myles Loyd described two methods for power removal: an onboard generator and a tether lift. Leo Goldstein proposed a third method: fast motion transfer using a separate belt.
In the systems of this type, one or more propellers and electrical generators are installed on a wing. The relative airflow rotates the propellers, which transfer the power to the generators. Electrical energy is produced and transmitted to the ground through an electrical cable that is laid along the tether. This system is best exemplified by Google’s Makani Power. This type of AWES faces challenges, such as heavy weight and the high cost of airborne generators and extensive safety requirements.
In tether lifts, an electrical generator is installed on the ground. There are two subtypes of tether lifts: those with and those without a secondary vehicle.
In the tether lift without a secondary vehicle, the tether slowly unwinds off a drum on the ground. This process rotates the rotor of the generator through a high-ratio gearbox. Periodically, the wing is depowered and the tether is reeled in. In some systems two tethers, connected to the kite’s edges, are used instead of one. Such systems face high forces and torques acting on the ground equipment. When increasing the power, these forces increase faster than linearly, which makes scaling up difficult.
In the other subtype of tether lifts, a secondary vehicle is used. This vehicle can be a carousel, a car or even a ship. The electrical generator is installed on the vehicle. The rotor of the electrical generator is pulled into motion by the carousel, the axle of the car or the screw of the ship. The secondary vehicle increases the cost of this type of system.
Kitegen, Italy, suggested designs in each of these subtypes. Some of the best known companies, working on systems with ground generator are Ampyx Power, Netherlands, and SkySails Power, Germany. TU Delft University of Technology has one of the strongest research programs in the airborne wind energy.
In this type of AWECS, an electrical generator is installed on the ground and a separate belt, which trails behind the wing, transfers the power to a sprocket on the ground. The sprocket rotates the generator’s rotor. The separate belt extends at approximately the speed of the wing. Since Power = Force x Speed, when the speed of the belt is high, the force is low. In this type of AWECS, the rotational speed of the main axle is high so the torque is low and no gearbox is required. This technology holds the most promise for future development. An article analyzing an airborne wind energy system of this type was published in the June, 2013 volume of Energy Journal. A quote from the article: “The economic analysis shows that the proposed system is 10 times less expensive than a conventional wind turbine with a comparable average power output.” Details are presented on AWELabs’ Technology page. Artist’s impression an AWES with fast motion transfer is shown on AWELabs’ home page.
In any AWES type, the wing can be flexible (that is, a kite) or rigid (that is, a glider). Last, but not least, is a control system. An AWES control system must ensure stable motion of the wing (or wings) in the energy harvesting cycle and accommodate wind changes and other weather conditions, such as fog and rain. The control system must ensure launch and retraction of the wings. The control system must also operate day and night. Airborne wind energy is sometimes referred to as high-altitude wind power or simply kite power.