Wind can be an unpredictable and unstable source of power, and high in the sky where it is more stable, it's difficult to exploit. Airborne wind turbines could provide a solution to this problem, but although the idea has been around since the 19th century, it's never been exploited on a larger scale. California's Makani Power aims to change that with its innovative flying wing turbine design. Wing 7 is essentially a cross between a UAV and a wind turbine that's tethered to a ground station from which it ascends to a height of around 1,300 feet (400m) and flies autonomously, generating up to 20-kilowatt of power in a 20mph (35km/h) wind.

Makani Power's designers have used carbon fiber to construct the 8 meter (26.2ft) wide Wing 7 prototype, keeping the weight down to 58.4kg (128.7lbs). The flying wing can move both vertically and horizontally due to its uniquely designed tail and rotors. It takes off vertically and after reaching the appropriate height, it becomes a sophisticated kite with onboard avionics that enable it to fly horizontally in crosswind circles. The electricity generated by its rotors is then transmitted down the electrically-conductive tether for storage purpose.

The circular path taken by the flying wing is designed to mimic the tip of a wind turbine blade (its most efficient part) and sweeps a much larger section of the sky than a conventional wind tower. This, along with less material and lower distribution costs, adds up to a more efficient renewable energy solution according to Makani.

Wing 7 is being tested in a series of flights this year and, with the backing of funding from the Department of Energy's Advanced Research Projects Agency-Energy (ARPA-E) and private investors including Google founders Larry Page and Sergey Brin, Makani Power hopes to develop a 1-megawat flying turbine - the Makani M1 - that's capable of operating at 1,800 ft (550m) altitude and bring it to market in 2015.

The video below presents a test flight made in July, that fully demonstrated Wing 7's power generation and autonomous flight modes:

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