RENEWABLE ENERGY:

Study maps best spots for harnessing high-altitude winds

Jet streams have enough wind to power the entire planet, so some far-sighted researchers are working on technologies -- high-flying kites and floating turbines -- that just might harness that power.

"High-altitude winds represent the largest highly concentrated form of renewable energy available on Earth, and if we could learn how to extract energy from winds and distribute it globally, we could potentially power all of civilization," said Ken Caldeira, a climate scientist with Stanford University's Carnegie Institution.

Best suited for high-altitude wind development are population centers in the eastern United States and East Asia, according to research by Caldeira and Cristina Archer at California State University, Chico.

In a study published recently in the journal Energies, Archer and Caldeira report using historical data to conduct the first global assessment of high-altitude wind power.

They found the highest wind densities -- a measurement that accounts for both wind speed and air density at different altitudes -- about 6 miles above Japan and eastern China, the eastern U.S. coast, southern Australia and northeastern Africa.

Their research also studied the high-altitude wind profiles above the five largest cities in the world and found New York, Tokyo and Seoul to have significant potential because of their position below polar jet streams. Jet streams are meandering atmospheric currents with wind speeds of an order of magnitude faster than those on the ground.

Harvesting high-altitude wind requires relatively little space to produce large amounts of energy. The wind above New York City, for example, has enough energy potential in a square meter to meet an average American's power demands.

"Most other renewable energy sources require huge distribution systems -- solar farms on the ground surface or large wind turbine arrays," Caldeira said. "The highly concentrated nature of wind energy in jet streams requires relatively little area per person."

But that highly concentrated wind energy poses problems, as well, the study found.

"We originally thought, the perception was, that high altitude winds were persistent and steady. But they're not as persistent as we hoped to find," Archer said. "There are still a significant number of hours with too little wind."

And because the jet stream meanders, even regions with large energy potential would experience periods of intermittency.

To counteract that, Caldeira said, a high-altitude wind array would need back-up power for periods of low wind activity -- for instance, it would require a connection to a back-up plant, a continental-scale transmission grid or a huge battery that would store power in windy times.

Transfer technology needed

Still, the jet streams are far more reliable than winds on the ground, Caldiera said. They just do not yet provide an energy source that is as reliable as coal-fired power plants.

The handful of start-up companies working to harness high-altitude winds still need to figure out the best way to transfer energy generated miles above the Earth's surface to the ground.

"I think sooner or later we'll see people tapping into this resource, but right now, the technologies don't really exist to get winds from the jet streams -- maybe to go halfway up," Caldeira said. "The reason is this: Basically, you have to tether something like a kite."

Moreover, the kite tethers would need to be able to transmit mechanical energy or electricity to the ground without using too much energy in the process.

But the scientists remain optimistic.

Archer said demonstration plants could be operating within five years. And with proper investment, she said, the technology could be commercial-ready in a decade.

"Prototypes are currently working at smaller scales," Archer said. "It's very, very exciting, and we're making progress like crazy. I don't think it's the solution to the energy problem -- not only this. But this will be part of the solution."

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