Green energy start-up Makani Power has come a long way from its origin as a gleam in a some kite-surfers' eyes. Friends Corwin Hardham, Don Montague and Saul Griffith founded the company in 2006. Its goal: to generate energy from thin air. Makani's tethered carbon fiber wings fly between 800 and 1,950 feet above the ground while their mounted rotors generate electricity, serving as a wind turbine in mid-air. The benefits of height are the powerful and more consistent winds found at high altitude.

Here, Makani's airborne wind turbine sits on the runway prior to a test flight in Alameda, California.

The unmanned wings fly in a circular flight path, controlled by a computer. Electricity generated by the rotors travels down the tether, eliminating the need for the massive cement towers holding traditional wind turbines.

In February 2012 the turbine performed three transitions in and out of crosswind flight as required the US Department of Energy's ARPA-E grant. In crosswind flight mode the turbine flies in large, vertical circles. The main wing creates lift, keeping the wing aloft, while an onboard computer keeps it flying in a circle. Meanwhile, four rotors on the main wing slow the wing down while generating power, like the brakes on a hybrid car. For more on how the wings keep aloft, click here.

The turbine's hybrid rotors are a clever innovation—they can either generate energy as a turbine or apply thrust like a propeller. Thus the wing can stay aloft if the wind dies, by virtue of its propellers.

Makani is developing software to better handle the vagaries of wind power, including high wind gusts. Presently the company's airborne wind turbine can generate 30 kilowatts, compared to the approximately 600 kW a traditional wind turbine might produce. However the company says their planned first commercial product will be a 600 kW utility-scale airborne wind turbine.

Testing Team Lead Paula Echeverri installs electronics in the wing as she prepares it for a new series of test flights focusing on launching and landing to a ground station. She is assisted by her brother, Santiago Echeverri, an intern who is a candidate for a mechanical engineering degree in Colombia.

Makani engineers work on an early prototype of the ground station, from which the wing launches and lands. In order to provide electricity on the scale of wind farms, hundreds of airborne wind turbines will need to be launched and operated, necessitating such a central hub.

Shown here, the launch platform Makani is currently developing for use during test flights.

The company claims that four times more land in the U.S. could be suitable for airborne wind turbines than is suitable for the conventional kind. In addition, offshore areas that are too deep for conventional turbines can also host a flying turbine, tethered to a lightweight floating foundation in waters up to 90 meters deep.