Powerful Ideas: Miles-High Kites Could Generate Electricity

The sky might literally be the limit for wind power — rotors spinning miles high could help supply electricity worldwide

“There is a huge amount of energy available in high-altitude winds,”

said researcher Ken Caldeira at the Carnegie Institution’s Department

of Global Ecology in Stanford, Calif. “These winds blow much more

strongly and steadily than near-surface winds, but you need to go get up miles to get a big advantage. Ideally, you would like to be up near the jet streams, around 30,000 feet.”

All told, if wind turbines

miles above the planet were tethered to 10 percent of the world’s land,

there is enough energy in these jet stream winds to meet world demand

100 times over, researchers said.

Jet streams are meandering belts of fast winds at altitudes between

20,000 and 50,000 feet. They shift seasonally, but are otherwise

persistent features in the atmosphere. Jet stream winds are generally

steadier and 10 times faster than wind near the ground, making them a

potentially vast and dependable source of energy.

But how to capture the wind so high?

Kites and tethers

A number of technological schemes have been proposed to harvest energy from these high-altitude winds, including tethered, kite-like wind turbines

lofted miles high. Up to 40 megawatts of electricity could be generated

by current designs and transmitted to the ground via tether.

Using 28 years of weather data, the researchers developed the first-ever global survey of high-altitude wind energy.

“We found the highest wind power densities over Japan and eastern

China, the eastern coast of the United States, southern Australia, and

north eastern Africa,” said researcher Cristina Archer, an atmospheric

scientist at California State University in Chico.

These specific areas generate roughly 10 kilowatts per square meter

or more. “This is unthinkable near the ground, where even the best

locations have usually less than one kilowatt per square meter,” Archer

added.

The analysis also looked at some of the world’s largest cities:

Tokyo, New York, Sao Paulo, Seoul and Mexico City. New York proved a

prime location, as did the East Asian cities.

“For cities that are affected by polar jet streams such as Tokyo,

Seoul and New York, the high-altitude resource is phenomenal,” Archer

said. “New York, which has the highest average high-altitude wind power

density of any U.S. city, has an average wind power density of up to 16

kilowatts per square meter.”

Tokyo and Seoul also have high wind power density, as they are both

affected by the East Asian jet stream. Since Mexico City and Sao Paulo

are located at tropical latitudes, they are rarely affected by the

polar jet streams and only occasionally by the weaker sub-tropical

jets. As a result they see lower wind power densities than the other

three cities.

Issues remain

Another issue is whether implementing such devices on a wide scale

could alter general air circulation patterns and thus impact local and global climate.

Their simulations hint that if carried to unlikely extremes, blanketing

the entire planet with such devices would cool the Earth’s surface,

reduce precipitation and boost sea ice levels. However, if deployed at

levels comparable to total global electricity demand, there seemed to

be no detectable effect on the climate even after 70 years.

Also, fluctuating wind strength still presents a challenge when it

comes to exploiting this energy source on a large scale, just as it

does on the ground.

“While there is enough power in these high altitude winds to power

all of modern civilization, at any specific location there are still

times when the winds do not blow,” Caldeira said. Even over the best

areas, the wind can be expected to fail about 5 percent of the time.

“This means that you either need back-up power, massive amounts of

energy storage, or a continental or even global scale electricity grid

to assure power availability,” he added. “So, while high-altitude wind

may ultimately prove to be a major energy source, it requires

substantial infrastructure.”