Australian engineer Bryan Roberts wants to build a power station in the sky – a cluster of flying windmills soaring 15,000 feet in the air – but is having trouble raising enough money to get the project off the ground.

After 25 years of research, Roberts has designed a helicopter-like rotorcraft to hoist a wind turbine high into the air, where winds are persistent and strong. The craft, which is powered by its own electricity and can stay aloft for months, feeds electricity to the ground through a cable.

Roberts, a professor of engineering at the University of Technology, Sydney, believes there is enough energy in high-altitude winds to satisfy the world's demands. Wind-tunnel data suggests a cluster of 600 flying electric generators, or FEGs, could produce three times as much energy as the United States' most productive nuclear power plant.

Roberts has teamed up with Sky WindPower, a San Diego startup that is trying to commercialize his invention.

The company has Federal Aviation Administration approval to conduct tests of the technology in the California desert, but needs $3 million to build full-size flying generators. The company is having trouble raising the cash because there isn't likely to be an immediate return on investors' money.

High-altitude winds could provide a potentially enormous renewable energy source, and scientists like Roberts believe flying windmills could put an end to dependence on fossil fuels.

At 15,000 feet, winds are strong and constant. On the ground, wind is often unreliable – the biggest problem for ground-based wind turbines. "For FEGs, the winds are much more persistent than on ground-based machines," said Roberts. "That's part of the benefit, more power and greater concentration."

Ken Caldeira, a climate scientist at Lawrence Livermore National Laboratory, said tapping into just 1 percent of the energy produced by high-altitude winds could satisfy a lot of the world's power needs.

"It's absurd that all this time we have turned a blind eye to the energy right above our heads," he said. "High-altitude wind power represents the most concentrated flux of renewable energy found on Earth."

At certain locations, the efficiency of a flying generator can be as high as 90 percent, three times higher than its grounded counterpart, according to Sky WindPower.

At this efficiency, FEGs could become the nation's cheapest source of electricity, with an estimated cost per kilowatt hour of less than 2 cents, about half the price of coal, according to the Power Marketing Association.

Having conducted tests with models, Sky WindPower wants to scale up Roberts' experiments and produce a commercial-sized flying windmill with four rotors. The rotorcraft will go into the first layer of the atmosphere, called the troposphere. Sky WindPower estimates the craft will produce 200 kilowatts per hour of electricity in an area that at ground level would produce none because of a lack of wind.

Since strong high-altitude winds exist in many locations, the company's hope is to find sites 10 miles by 20 miles in size that are not currently used by commercial planes and turn them into restricted airspaces. Once in the air, the FEGs' roll and pitch would be controlled to catch the wind most effectively. Sky WindPower intends to use GPS technology to maintain the crafts' vertical and horizontal location to within a few feet. The craft will be brought to ground once a month or so for maintenance checks.

The project has already received FAA approval and needs only to finalize a test site. Currently the company favors somewhere in Southern California. The company declined to be specific, saying it has not yet applied for local permits.

"Our desert test site does not have as good winds as future intended operational sites," said David Shepard, president of Sky WindPower. "But starting there will enable us to proceed to more-difficult conditions with less risk."

However, the company has not yet raised the capital to build the craft. Shepard said he expected the money would be found.

"We do have reason to expect that we will obtain the funding necessary to carry out our intended demonstration," he said. "I have reason to be optimistic."

Caldeira, whose high-altitude wind energy graphs can be found on Wind SkyPower's website, said he was disappointed but not surprised the company is having trouble raising money for testing.

"Investors tend not to put money into a project that is risky or won't pay within a few years," he said. Caldeira said there are lots of obstacles that scare investors: testing, obtaining local permission and quelling fears of possible danger.

"Even if this is a far-out possibility, our society is remiss in not vigorously investigating the potential for high-altitude wind power," he added.

The lack of initial investors is expected by some economists.

"The high risk of investment is associated in terms of length before you would see a return," said Stanley Stephenson, an economist for Litigation Economics, an economic and legal consulting firm.

Stephenson said there is also the fear that larger organizations, even oil companies, could enter the market with alternative energy technologies, which makes it more difficult to find investors who are gung-ho for small venture startups.

But Eric Becker, vice president of Trillium Asset Management, a firm that specializes in socially responsible investing, believes that entrepreneurial capital for projects such as Sky WindPower will soon start to flow.

Becker cited EnviroMission, an alternative-energy company in Australia that is building a kilometer-high solar tower.

"It's as far-fetched as tethered wind turbines and they've managed to go public and are on the verge of booking orders," said Becker.

After low-altitude and wind-tunnel testing, Roberts doesn't believe his invention is far-fetched at all. "It has all been properly done in an academic sense. I've never had any criticism in the principle," he said.

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