Twenty-five-knot gusts sweep across the Pacific, scooping up kiteboarders and flinging them into the sky. Atop a seaside bluff, the wind races through the grass in long lines, the prairie version of ocean waves, and buffets a panel delivery truck parked at the end of a dirt road. The logo in peeling paint on the side reads TOM'S QUALITY SNACKS… FOR EVERY TASTE. But there are no chips or Choco Tacos inside. Instead, four young men sit elbow to elbow, staring at computer screens filled with code. They act like an FBI surveillance team awaiting the big sting, until one of them jumps out the back and grabs what looks like a large model airplane.

He walks downwind, carrying the plane. It's nearly as large as he is. A voice from snack-truck mission control crackles over his radio—"launch when ready"—and he heaves the plane into the sky. The propeller hums. A pilot standing nearby maneuvers the craft with a remote control, but it's obvious this is no hobby flight. Rather than cruising aimlessly, the plane carves identical circles. A tether connects it to the ground—and after a few minutes, the pilot puts his controller down and software takes over. The plane is flying itself.

Of all the things you might guess are taking place, testing a potent new ­method for generating clean power would probably rank near the bottom of the list. But here on the coast, just north of Santa Cruz, Calif., that's exactly what is ­happening. These engineers from Joby Energy are developing a technology known as airborne wind. Like traditional wind power, it employs spinning rotors to generate electricity. But the similarity ends there. Joby's engineers want to ditch the bulky support towers of wind farms. They want to teach windmills to fly.

The plane climbed, driven by its propeller, until its tether was taut. But now, the wind alone, racing over the wings, provides sufficient lift, freeing the propeller to function as the rotor of a wind generator. Joby is building models 10 times the size of this research prototype, some with up to 12 rotors. In a fully deployed system, the electricity generated would be routed down the tether and into the grid.

The airborne wind industry is a gnat next to B-52s like hydropower and coal. But the sector is booming, with Joby and its closest rival, Makani Power, leading a race among more than a dozen startups. The companies have poured an estimated $50 million into R&D, and they are backed by Silicon Valley venture capitalists in search of the next big thing, as well as by ARPA-E, the Department of Energy agency that funds cutting-edge research. The promise of airborne wind has even wowed Google co-founders ­Larry Page and Sergey Brin, who plunked $20 million into Makani. Ken Caldeira, a senior climate scientist for the Carnegie Institution for Science at Stanford ­University, studied airborne wind relative to other energy options and came away impressed. "Airborne wind is one of the few potential sources that can supply power on the scale that civilization needs," he says.

Airborne wind farms might have the same number of turbines, the same ­distance apart, as today's terrestrial ones. But they would fly on tethers 1000 feet or higher in the sky. Because the wind is stronger and more consistent there, ­power generation would no longer be limited to the world's gustiest places, making the technology widely deployable. "Think of an airborne turbine as just a turbine on a really tall tower—without needing to pay for the tower," says ­JoeBen Bevirt, the founder of Joby Energy.

High-yield. Low-cost. Clean. It all sounds great, but for these promises to pan out, the turbines must ultimately be able to take off safely, fly for hours or days and land without a human pilot—critical capabilities that are unproven and years away from commercialization. "The people doing airborne wind are visionaries," says Fort Felker, the ­National Renewable Energy Labora­tory's leading expert on wind power. "But none of them has brought a product to market that has the safety and reliability requirements of flight vehicles."

Inside the snack truck, engineer ­Henry Hallam tells me, "The plan for the day is to do some endurance testing and autonomous flight. If all goes well, it will be really boring." But the wind is too spirited for boring. On the fourth test, the plane is rocked by a pop; it belches a ball of fire, zigzags and lands hard. Bevirt vaults from the truck; engineer Greg Horn follows with a fire extinguisher.­ The plane, fortunately, is fine, and it doesn't take long to figure out what ­happened. The model is a testbed for studying flight control systems, not ­energy production, but the wind was so strong, the motor controller couldn't brake the propeller sufficiently. "We ­generated so much power, we melted our wires," Horn says. Bevirt turns to me with a smile. "It gives you a sense of how much energy is up there, huh?"

Joby's headquarters are tucked into the redwoods of the Santa Cruz Mountains, not far from the test site. The lodge-like main building is encased by tall windows and trussed with dark ­wooden beams; outside, there's a deck with barbecues and umbrella-topped tables, a shady lawn and a large organic garden. The place is patrolled by friendly dogs and catered by gourmet chefs, creating a vibe that's less corporate headquarters and more high-end yoga retreat.

Bevirt is pinballing around the grounds when I arrive. He jogs downhill to the warehouse, calling out questions to colleagues and striding between lathes, mills and other shop tools. The 37-year-old has been on the go from an early age: As a high school cycling ­fanatic, he designed and built several bikes; during college he worked as an engineer and saved $50,000, which he invested in the stock market.

By the end of the 1990s, after earning a master's degree in mechanical engineering at Stanford University, he cashed out a $500,000 portfolio and seeded his first business, the laboratory-equipment manufacturer Velocity11, and then Joby, Inc., which makes the GorillaPod line of flexible tripods. These successes gave him the capital to launch Joby Energy, as well as an aviation company. Joby Energy is a project of environmental passion, but it's also a business. "Energy is just a commodity—one electron is no better than another," he says. "What matters is the cost."

Ground-based wind turbines don't spin at full speed every minute of every day. Sometimes the wind blows weakly; sometimes­ not at all. That's why conventional ­windmills generate only up to about one-third of their theoretical full power. But the wind where many airborne companies want to fly, at an altitude of about 1300 feet, ­typically blows more consistently and one and a half to three times faster than at the earth's surface. That means airborne wind could run at a projected capacity factor of 70 percent, Bevirt says—twice the efficiency of terrestrial wind.

Many experts, however, are not yet convinced. The National Renewable Energy Laboratory's Felker says the ­airborne wind industry probably does have an advantage in capacity factor when its machines are in the air. But land-based turbines can operate roughly 98.5 percent of the time, reliability that flying turbines could not match. "There's no example in the history of the universe of a flight vehicle being available 98.5 percent of the time," Felker says. (Bevirt says that Joby's projections assume ­airborne turbines will be grounded 5 to 20 percent of the time.)

Everyone agrees that airborne wind needs more R&D. The catch-22 is that proving new technologies takes money, yet investors are wary of the unproven.­ Furthermore, the industry's path to regulatory approval might be tortuous. Elizabeth Ray, a spokeswoman for the Federal Aviation Administration, said at a recent airborne wind energy conference that flying turbines will have to elbow their way into a sky already crowded with cellphone towers, buildings and aircraft. Airborne turbines, in a perfect world, might one day operate at 30,000 feet to tap the powerful jet stream. Flying at those altitudes would make FAA approval even tougher. "It's all competition for the same finite resource," Ray said—meaning airspace.

Meanwhile, engineers are trying to create flying machines such as the world has never seen—part helicopter, kite, plane and robot. They must be autonomous, because labor costs for ground-based­ pilots would wipe out the technology's­ economic advantages. They must be reliable, because life-endangering­ crashes could scuttle the industry. (For this reason, Bevirt recommends early sites be established in ­uninhabited areas or offshore.) The public will need to become comfortable with the idea of turbines filling the sky, just as it did a century ago with planes, which are now essentially ignored.

Inside Joby headquarters, a dozen fly ing contraptions dangle from the ceiling, a visual timeline of corporate evolution. There are biplanes, triplanes and what looks like a giant, flying game piece from Trivial Pursuit. Engineer Jeff Gibboney describes a recent Joby design—a 38-foot-long biplane with no fuselage or tail. "It's like the National Air and Space Museum in here," I say, admiring the collection. "Yeah," Gibboney replies. "Only weirder."

Dawn Breaks over the San Francisco Bay. As the soaring bridges fill up with cars, Corwin Hardham paddles his surfboard toward golden clouds. Instead of horns, he hears lapping waves; instead of red taillights, he spots a seal poking its head above the swells. Hardham is the co-founder of Makani Power, and this is his Friday-morning commute. I paddle behind him, precariously balanced on a loaner board.

Once a week Hardham spends rush hour this way because it's greener than driving. Mainly he just loves being out on the water. In his late teens Hardham ­considered becoming a professional windsurfer, a pursuit that influenced his career in ways no one could have predicted. As an undergrad at MIT, he befriended another technically minded windsurfer, the now-renowned inventor and PM adviser Saul Griffith. After graduate school, they launched Makani ­Power with a third friend, Don Montague, a former professional windsurfer and kiteboarder. "Wind sports give you a visceral sense of how powerful the wind is," Hardham says.

An hour later, we reach the island of Alameda, where Makani—minus ­Griffith, who has moved on to other endeavors—has set up shop in the air traffic control building of a decommissioned naval base. We swap wetsuits for work clothes, and Hardham drives us onto the cracked asphalt of the runway, where preflight tests are underway.

Hardham parks by a firetruck that will serve as the anchor for a tether extending 500 feet to Makani's prototype. Wing 6 has a 10-foot-wide airfoil and a three-pronged body. The tail stabilizer is aligned vertically for the hovering takeoff but will switch to horizontal for flight. "Either mode is relatively straightforward," Hardham says. "The challenge is making a wing that does both."

Altitude aside, the true magic of airborne turbines is that they move. Like a stunt kite on a beach, they zip around in relation to both the ground and the wind direction. This technique, known as crosswind flight, makes it possible to capitalize on net wind speeds that are much higher than the ambient speed alone. The ramifications for the fight against climate change could be huge.

To cap the level of atmospheric CO2 at roughly double what it was before the Industrial Revolution—a common target used by climate scientists—"you'd need something like 15 terawatts of primary power from carbon-neutral ­sources," says the Carnegie Institution's Caldeira. "That's more or less saying we need to build another energy system as big as the entire current one." Generating massive amounts of power from traditional solar and wind would require a massive amount of space. "To supply even 20 ­percent of the electricity in the U.S. from terrestrial wind," Hardham says, "you would have to cover the state of Kansas with 1.5-megawatt turbines spaced as closely as you could."

Caldeira and Cristina Archer, an ­airborne wind expert at California State University, Chico, calculated that ­airborne wind could be far more efficient. "Airborne wind could potentially produce 18 terawatts of electricity, which is more than enough to power modern civilization without adverse effects on climate," Caldeira says. Supplying 18 terawatts would require millions of ­airborne turbines, but Caldeira says his point is not that such a goal is realistic; rather, it's that large-scale airborne wind ­production is feasible. He thinks the industry could generate 10 percent of the planet's power, making it a major contributor to the overall energy mix. A wind farm with 800 airborne 1-megawatt turbines, he says, could power 250,000 homes.

On the runway in Alameda, Makani is working on the transitions between flight modes. No company has completed a fully autonomous flight yet, though both Joby and Makani have prototypes that need pilots only for takeoffs and landings. In the fall of 2010, Wing 6 transitioned from a hovering phase to its flight phase and back to hovering. "That's an important milestone," Hardham says. "You can see one craft doing all the ­necessary flight modes."

The controllers work through their checklist. "Final wind check," comes a voice over the radio. "We have 2.4 meters per second. Direction good." With a whine like angry mosquitoes, and the city of San Francisco twinkling in the background, Wing 6 takes to the sky.

This article appears in the March 2011 issue of Popular Mechanics.

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