Pictures: 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. (Her interview is in the author's book, Build Your Own Small Wind Power System.)

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.

(Related Story: "Helix Collapse Fails to Crush Hope for Vertical Wind Turbines")

Published September 24, 2012

Inspired by a Googler Hobby

Photograph courtesy Makani Power

Resembling a drone aircraft on a string, the Makani Airborne Wind Turbine takes flight at its test site, the decommissioned U.S. Navy air station at  Alameda on San Francisco Bay.

By eliminating 90 percent of the material associated with a conventional wind turbine-largely by getting rid of the tower—the designers say they hope to reduce cost while accessing stronger winds.

The winged device is tethered to the ground and flies in large vertical circles at altitudes between 800 and 1,950 feet (250 and 600 meters). Its four wind turbines rotate as the craft moves. According to Makani Power, the speed of the craft increases along with wind speed.

Makani Power's website says the company is developing a 600-kilowatt (kW) prototype. That's considered the size of a medium commercial wind turbine; for comparison, a 600 kW land-based turbine installed in 2009 at University of Maine at Presque Isle generated 680,000 kilowatt-hours of electricity in its first year, enough to power about 60 average U.S. homes. But an airborne wind turbine might deliver more or less power, depending on the boost of stronger, more consistent winds or the cost of trickier operation.

Makani was founded in 2006 and received $10 million in initial start-up capital from Google's foundation, plus support from the U.S. Department of Energy.

One of Makani's three co-founders, Corwin Hardham, has told reporters he was inspired by his hobby of kite-surfing. It's not a coincidence that Alameda has a beach that is popular with Bay Area kite-surfers. It also isn't far from Google headquarters; the company's founders, Sergey Brin and Larry Page, are known to be avid kite-surfers.

(Related Story: "Sizing Up Wind Energy: Bigger Means Greener, Study Says")

Published September 24, 2012

Computer-Driven Power Kite

Photograph courtesy KiteGen

The semi-circular power kite of the KiteGen device curls in the wind, rotating a vertical shaft to generate electricity in Italian inventor Massimo Ippolito's system.

Like the Makani founders, Ippolito says he was inspired by kite-surfing. Active computer controls help keep the kite, with its airfoil shape, optimally oriented into the winds, Ippolito said in an interview for Build Your Own Small Wind Power System. Each airfoil is controlled by two lines, much like a stunt kite, as the shaft rotates in a circular pattern.

Ippolito said he hopes to get his airfoil 2,625 to 3,281 feet (800 to 1,000 meters) above ground—about six to eight times the height of today's popular ground-based 1.5 megawatt (MW) wind turbine. By leaving all the expensive generating equipment on the ground, Ippolito said he hopes to reduce costs, risks, and maintenance time. He said he is trying to develop KiteGen power plants with a capacity of 1,000 MW (about the same as a big coal plant).

In their new paper, Caldeira and his colleagues considered whether there are any significant geophysical limitations to considering wind energy as a major primary energy source (as coal is today). Would the increased surface drag of large-scale wind energy deployment have harmful impacts on the Earth's climate? Several studies in recent years have looked at large-scale wind farms and their potential to impact temperature or precipitation.

(Related Story: "Planting Wind Energy on Farms May Help Crops, Say Researchers")

Through climate modeling, Caldeira's team concluded that indeed, if wind power were deployed at a level about 100 times higher than civilization's current energy use, profound climate changes would occur. The drag created by the turbines would alter Earth's circulation pattern and impede heat transfer into polar regions. But at one percent of that level—if wind turbines were deployed to satisfy all of the energy civilization is currently consuming, from car-fueling to electricity—the resulting wind drag would alter the global climate by only a tenth of a degree Celsius and change precipitation rates only by one percent. In other words, says Caldeira, "Environmental consequences would be small and manageable."

Wind turbines, of course, are not alone among human developments that can cause wind patterns to shift. "This is also the case with converting forests to croplands, building cities, and many other changes," said Caldeira. "Any time you toy with drag in the atmosphere you will change circulation somewhat."

For next steps, Caldeira said his team is modeling airborne wind turbines on a theoretical planet that is entirely covered by water, to simplify the calculations and try to better understand the complex physics, "which are more complicated than we thought." Because airborne wind also seems to have a cooling effect, the team is also looking into how aloft turbines could be purposefully used, as a form of geoengineering, to cool down the over-heating planet. (See video of Caldeira discussing his research.)

Published September 24, 2012

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.

Published September 24, 2012

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.

Published September 24, 2012

A Serpent in the Sky

Photograph courtesy Doug Selsam

Slithering in the wind, the Sky Serpent is a chain of small wind turbine blades lofted into the air by balloons. California-based wind turbine inventor Doug Selsam hopes a version of his prototype can be carried high into powerful winds.

Selsam says his Sky Serpent was the only working prototype at the first world Airborne Wind Energy Conference at California State, Chico, and Oroville, California, in 2009. "This in a field against Boeing, Honeywell, etc.," he said in an email. The device garnered one of Popular Science magazine's "Invention of the Year" awards in 2008.

Selsam is a former heavy metal rocker, but he said his multi-rotor turbines use less material and cost less to make than conventional designs, yet can provide the same amount of power. Normally, adding more rotors can be a problem if the blades cause interference in the wind, but Selsam said he is able to space them carefully to avoid this.

An independent test by wind expert Paul Gipe in 2008 and 2009 was not completed because of technical difficulties, but Gipe reported that the Selsam machine was "powerful" and has said it may be worthy of additional study.

Selsam envisions long streamers of multirotor turbines floating offshore and over deserts. He has received funding from the California Energy Commission, and has tested more than a hundred different wind prototypes. He even made a demonstration kite-lofted turbine for $20 and "an afternoon's work" to prove the concept.

"Regular electric wind turbines started small and were produced in great numbers at the smaller scale for almost a century before we had the experience to start making them bigger, so my take is that an airborne wind energy product that can deliver any amount of power, economically, will be a good first step.

"The resource is there and no physical laws prevent airborne wind energy. We (humanity) just have to step up to the plate," Selsam said by email.

Published September 24, 2012

Next: Helix Wind Collapse Fails to Crush Hope for Vertical Turbines

Photograph by Robert Kaye

Published September 24, 2012