The world's strongest winds race high in the sky, but that doesn't mean they're out of reach as a potentially potent energy source.
Flying, swooping and floating turbines are being developed to turn high-altitude winds into electricity.
The challenges are huge, but the potential is immense. Scientists estimate the energy in the jet streams is 100 times the amount of power used worldwide annually.
Cristina Archer, an atmospheric scientist at the California State University in Chico, said there's "not a doubt anymore" that high-altitude winds will be tapped for power.
"This can be done, it can work," she said.
The question is, when? Some companies project their technology will hit the market by the middle of the decade, but Fort Felker at the National Renewable Energy Laboratory says the industry is 10 years away from making a meaningful contribution to the nation's electricity demands.
No company, for instance, has met the basic requirement of demonstrating its turbine can safely fly unsupervised for prolonged periods of time.
High-altitude wind power is similar to ground wind in the 1970s — facing questions but soon to prove its viability, said PJ Shepard of Oroville, California-based Sky WindPower, which is developing a "flying electric generator."
"It's kind of like the adjustment folks had to make when the Wright brothers started flying airplanes," she said.
The lure of high-altitude wind is simple: Wind speed generally increases with its height above the ground as surface friction diminishes. Each time wind speed doubles, the amount of energy it theoretically holds multiplies by eight times.
The world's most powerful winds circulate in the jet streams, which are found four to 10 miles (16 kilometers) off the ground and carry winds that regularly break 100 miles per hour (160 kph).
The dream is to eventually tap the jet streams, but high-altitude wind companies are focusing for now below a 2,000-foot (600-meter) ceiling, above which complex federal air-space restrictions kick in. Adam Rein, co-founder of the Boston company Altaeros Energies, said his company calculates winds at the 2,000 foot (600-meter) level are up to 2½ times stronger than winds that can be reached by a typical 350-foot (105-meter) land turbine.
High-altitude wind advocates say their smaller, lightweight turbines will be far cheaper to build and deploy than windmills with huge blades and towers that must be drilled into land or the sea floor.
Those savings would mean inexpensive energy. With wide-scale use, advocates see a range of prices, from something comparable to land wind's current 9 or 10 cents per kilowatt hour down to an astonishingly low 2 cents per kilowatt hour.
"They are projecting crazy numbers," Archer said. "I'm not saying that it's true. ... But it's really the lowest, the cheapest energy source, possibly."
As the turbines eventually aim higher, advocates say there are plenty of remote and offshore no-fly areas where they won't interfere with aircraft and have minimal interaction with people.
Still, any nearby residents must be convinced there's no danger from falling turbines while accepting a view that includes flying objects attached to long tethers that carry the energy to the ground. High-altitude turbines also won't escape the various environmental concerns that face conventional turbines, such as their effect on bird flight.
And each turbine concept awaits extensive testing on its reliability, durability and effectiveness, said Felker, director of NREL's National Wind Technology Center.
Keeping the turbines operating autonomously over long periods in changing weather may be the biggest obstacle, Felker said.
"You have powerful reasons to stay aloft as much as possible," he said. "What do you do when a thunderstorm comes by? Do you recover (the device)? Do you land? ... How do you operate in the real world?"
Different companies have proposed answering that question in different ways.
A "kite" being developed by Makani Power of Alameda, California, looks like a glider with four high-speed rotors that launch it into flight, then switch modes to generate electricity that's carried down the tether.
An onboard computer steers the kite in a wide circle, mimicking the path of the tip of a giant wind turbine blade. That allows the device to interact with a larger wind area, increasing the amount of energy it can capture in the same way increasing the size of a sail increases the potential speed of a sailboat, said Damon Vander Lind, Makani's chief engineer.
The kite can also stay up in high winds, and power itself to land in low winds, Vanderlind said.
"It lets us make a very reliable system, something operators can just plug in and use," he said.
Sky WindPower's generator has four rotors, each 35 feet (10.6 meters) in diameter, that transmit power down the tether. It's built to hover, rather than swoop or circle, Shepard said. While moving turbines can actually fly faster than the wind by flying crosswind, they can't fly as high, she said.
"We can get up a little higher than they can. We can get to higher velocity winds and make up for it," Shepard said.
Altaeros is developing a stationary turbine that sits inside a 60-foot (18.3-meter) tall, helium-filled shroud that acts like a wind funnel. Similar blimp-like devices, called aerostats, have long been used to keep heavy equipment aloft, such as government surveillance radar tethered up to 15,000 feet (4,570 meters) above U.S. borders.
"Our view is that our approach is less risky, because we're using a technology that's been out there for decades," Rein said.
Altaeros has big aims — no less than "making an impact on the global energy crisis," Rein said — but it's starting small. Its 10 full and part-time employees share a building with seven other start-up companies in a former wool manufacturing factory in South Boston.
Rein notes that before it tries to bring the untapped power of high-altitude wind to thousands, his company is first developing a system that could bring power to about 40 homes in remote areas.
"You start smaller... and then you scale up over time," Rein said. "We think that approach makes a lot of sense."