Technology Review – The Makani Airborne Wind Turbine consists of several turbines attached to a wing, which is tethered to the ground. In flight, the vehicle takes essentially the same path as the tip of a wind turbine’s blade, following a circle perpendicular to the wind direction. Thanks to crosswind aerodynamics—which produce the rapid circular movements familiar to anyone who has flown a kite on a gusty day—the apparent wind speed hitting the rotors can be as much as 10 times the actual wind speed.
Because the wing makes more efficient use of the wind than a fixed turbine, Hardham says, and is made of fewer, lighter materials, it should produce lower-cost energy. Maintenance can occur on the ground rather than at the top of a wind-turbine tower.
High-altitude wind harvesting still faces skeptics. “It’s a really interesting idea with potentially significant benefits, but we’re early in the process of sorting out whether it’ll work,” says Fort Felker, director of the National Wind Technology Center at the National Renewable Energy Laboratory, in Golden, Colorado. However, he says, reliability, safety, and economics are all concerns. Aerial vehicles designed to harvest wind energy need to be flying most of the time—and there’s always the risk they could “land on a school bus,” says Felker.
Test flight: A time-lapse photo shows the flight path of an airborne wind turbine. The vehicle is tethered to a converted fire engine (lower left) during a test in a remote area of Sherman Island, California.
The company has carried out a series of test flights, including a flight this fall during which the carbon-fiber prototype with a 26-foot wingspan generated five kilowatts of power. Within two years the company hopes to have an 88-foot wing that generates 600 kilowatts—around a third what a large conventional wind turbine can generate. A gargantuan wing to generate five megawatts is on the drawing board.
The company’s project has drawn some interest from funders. Google has put $15 million into the company, and in September 2010, Makani won a $3 million grant from the Department of Energy’s ARPA-E program, which funds high-risk ideas that could lead to what the agency calls “transformational and disruptive energy technologies.”