Flying Wind Turbines Reach for High-Altitude Power: Circling to Get Ahead

Photograph courtesy Doug Selsam

The KiteGen power kite is designed to fly in figure-eight patterns, according to illustrations on the company website.

Another pioneering system that has garnered some attention has been under development since 2005 by Kanata, Canada-based Magenn Power. Its 100-kW device, the Magenn Air Rotor System (MARS), is a helium-filled mini blimp designed to float up to 1,000 feet (305 meters).  (A land-based wind turbine of that capacity would be considered among the largest of small wind turbines.)

Wind makes the blimp's cylindrical core rotate around a horizontal axis, which generates electricity. The juice is then sent down the tether, according to the company.

Magenn claims the helium keeps the device especially stable in high-altitude winds, and says it has met U.S. Federal Aviation Administration guidelines for safety. Company marketing materials promote the product as ideal for remote applications like oil rigs and wilderness cabins, although it has not been released yet.

Some are skeptical of the company's claims. Alternative wind turbine designer Doug Selsam says the MARS system "takes the least efficient turbine type known and makes it more expensive and less efficient, by taking it into the air, with the balloon vastly increasing swept area without increasing power."

Magenn did not respond to a request for comment left on the company's Canadian headquarters answering system. The U.S. phone number listed on the company's website is disconnected.

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Flying Wind Turbines Reach for High-Altitude Power: KitGen Airfoil

A Game of Tetherball
 

Photograph courtesy KiteGen

The KiteGen airfoil prototype dangles above its housing, with its high-tension wires reeled in. The start-up company is based in Chieri, Italy, near Torino.

KiteGen builds on some earlier prototypes and theoretical ideas. In the 1980s, Bryan Roberts, an engineering professor at the University of Technology in Sydney, Australia, tested a small prototype of a helicopter-like airborne turbine that he hoped would eventually fly to 15,000 feet (4,600 meters), where it would float on the strong winds and send energy down a very long tether. Roberts' idea lives on in his Oroville, California-based spin-off company Sky WindPower, which claims to be working on a Flying Generator.

Since the 1970s, airborne wind designers have toyed with a concept called the Laddermill, which is made up of a loop or loops of kites deployed at high altitude. By varying the "attack angles" of the kites, operators can theoretically get them to dive or soar, or fly in endless circles, all to transmit energy to the ground.

Dutch astronaut and physicist Wubbo Ockels published a 4-kW version of a Laddermill in 2007. In this proposal, a loop of kites would be lofted at a height of 0.62 mile (one kilometer). As the kites climbed, they would unspool a tether around a drum, which would drive a generator. When the line ran out, they would be angled to dive, and the slack line would be recovered-and then the kites would be sent back up for another cycle. According to the published proceeding in European Power and Energy Systems, Ockels' team successfully tested a 2-kW (a typical U.S. household routinely draws about 2 kW, not counting air-conditioning).

In a somewhat similar concept, the Italian start-up Twind Technology is working on a device made of two tethered balloons, each with an inflatable sail. The sails are alternately filled and stowed, to make the pair swing back and forth. The resultant motion of their tether can be used to saw wood or drive an engine, according to the company.

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Flying Wind Turbines Reach for High-Altitude Power: The Wild Windy Yonder

Photograph courtesy Makani Power

The landscape appears to pitch beneath the Makani Airborne Wind Turbine  during a test flight near the start-up company's headquarters in Alameda, California.

Moving a wind turbine from a 328-foot (100-meter) tower to 1,640 feet (500 meters) above the ground would tend to double the available wind speeds, and increase the available power eightfold, says Cristina L. Archer, a University of Delaware engineering professor and one of Caldeira's former post-docs.

"Above 2,000 meters [6,562 feet] you get rapid gradients of winds, with the jet streams [at 30,000 feet (9,144) meters] being [the] Mecca of winds," Archer said.

Even though airborne wind pioneers are currently aiming at altitudes far below the jet stream, they face significant technological challenges as they try to bring wind power down to the ground. Long runs of wire can be expensive and prone to tangling. The devices could pose a risk to air traffic or the environment. They would also have to be protected from bad weather.

Joe Faust, editor of the website Upper Windpower, said in an email that research is under way on alternative transmission methods, including beaming power via lasers or microwaves, although such solutions are far off. More immediately viable, perhaps, has been the work on nonconductive tethers that transmit power by applying their motion to generators or fluid pumps, or by operating saws or moving carts.


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Flying Wind Turbines Reach for High-Altitude Power

Turbines Ready for Takeoff

Photograph courtesy Makani Power

Like the wing of a propeller plane without a cockpit, a Makani Airborne Wind Turbine stirs the air in a California field where it is being tested to capture high-altitude wind power.

Anyone who has climbed a mountain, a tower, or even a tall tree knows that winds get stronger at greater heights. There's less drag resistance from objects on the ground. That's why wind energy prospectors typically weld their expensive turbines to high towers, because the most important factor in power production is how fast the wind blows past the blades.

But what if turbines could reel in the power whirling above the reach of those tall towers?

Airborne wind energy pioneers, from North America to Italy and Australia, aim to find out. The technology is still in its infancy, although Makani's system—pictured above—has received notable backing from Google's philanthropic arm and the U.S. government. The concept also gained support in a new study published September 9 in the journal Nature Climate Change, which focused on the steady, fast high-altitude currents, and concluded that there's enough power in Earth's winds to be a primary source of near-zero-emission electric power as the global economy continues to grow through the 21st century.

The study found that wind turbines placed on Earth's surface could extract kinetic energy of at least 400 terawatts (trillion watts), while high-altitude wind power could extract more than 1,800 terawatts. The latter is about 100 times greater than the world's current power demand, the authors noted.

"The upshot is that airborne wind starts to look a lot like solar power," said study co-author Ken Caldeira, a senior climate researcher at Stanford University's Carnegie Institution for Science. "It's a resource that is large relative to human demand, and harvesting it has to do with economics and engineering, not fundamental limitations of the resource." (Of course, wind power ultimately can be called a form of solar power, too, because it's uneven heating by the sun that drives the winds.)

Caldeira and his colleagues, however, had the luxury in their theoretical study of not worrying about the practical challenges of deploying airborne wind turbines. Research and development being done by Makani Power and others is aimed at developing a cost-effective system to bring that high-flying energy down to Earth.

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