Imagine a city floating above the clouds and powered by a fleet of inflatable turbines, each one turning wind into electricity.
While that city and its residents may still be in the realm of science fiction, the idea of airborne wind factories is slowly becoming reality. Several tech firms are trying out new designs to harness the potential energy of winds that travel faster at altitude than at ground level.
"Winds are up to eight times stronger at 2,000 feet than at the 300 feet with wind towers," said Adam Rein, co-founder of Altaeros, a Boston-based startup. "The amount of power you can harness increases by cubic factor of the wind speed, so double wind speed is eight times the power. It's really amazing."
The Altaeros design uses a turbine blade made of lightweight composite aluminum surrounded by a circular shroud of laminate material that both focuses the wind and keeps the turbine aloft.
"It's not so different than a sail or parachute," Rein said.
The device resembles a jet engine that is filled with helium. Altaeros is preparing to launch a one-third scale prototype in the few months from an abandoned Air Force base near the Maine-Canada border. The 100-kilowatt blimp, or aerostat, will eventually generate enough power for about 40 homes, but Rein says he’s not after the residential marketplace.
Rather he hopes his airborne turbines will power remote military bases, drilling camps or small villages off the grid that now rely on expensive diesel gasoline generators. Since the whole device fits into a shipping container, the design also is ideal for humanitarian operations after natural disasters when power supplies are sketchy at best.
"We think there is a strong need for a renewable energy solution," Rein said. "Imagine trying to erect a 300 foot tower that weighs 70 tons in Northern Canada or Alaska or Africa. It's not feasible."
The floating turbine design recently won the ConocoPhillips Energy Prize for Compelling Renewable Energy Projects, but it's not the only one out there.
Alameda, Calif.-based Makani Power has designed a kite-like airfoil that circles the sky to generate electricity through its tether, while Joby Energy of Santa Cruz is building a flying rectangular array of turbines that it hopes will generate two megawatts of power.
European firms are looking at flying turbines that generate power by the winding mechanism of their ground-based tether.
All of these floating wind turbines are above migratory routes for birds and bats, making their environmental impact less than ground-based turbines which have been targeted by conservation groups. But they do have to contend with federal rules designed to keep airspace open for general aviation and military aircraft.
Despite a few obstacles, experts say the field is open to inventors who can figure out how to make the electricity competitive with existing sources of energy.
"We are early in the development of the technology," said Fort Felker, director of the National Wind Technology Center at the National Renewable Energy Laboratory in Golden, Colo.
Felker says helium-filled laminate blimps have been used for many years by the Coast Guard to support radar devices that track drug-runners. But combining a turbine and a blimp is new.
"There are no machines up yet to get operational experience, and I'm sure there will be surprises along the way," Felker said. "It's not can you do it, it's can you do it at an acceptable cost."