Highaltitude Wind

The most energetic gales soar far over the tops of today's turbines. New designs would rise higher—perhaps even to the jet stream

Wind is solar energy in motion. About 0.5 percent of the sunlight entering the atmosphere is transmuted into the kinetic energy of air: a mere 1.7 watts, on average, in the atmospheric column above every square meter of the earth. Fortunately, that energy is not distributed evenly but concentrated into strong currents. Unfortunately, the largest, most powerful and most consistent currents are all at high altitude. Hoffert estimates that roughly two thirds of the total wind energy on this planet resides in the upper troposphere, beyond the reach of today's wind farms.

Ken Caldeira of the Carnegie Institution of Washington once calculated how wind power varies with altitude, latitude and season. The mother lode is the jet stream, about 10,000 meters (33,000 feet) up between 20 and 40 degrees latitude in the Northern Hemisphere. In the skies over the U.S., Europe, China and Japan—indeed, many of the countries best prepared to exploit it—wind power surges to 5,000 or even 10,000 watts a square meter. The jet stream does wander. But it never stops.

If wind is ever to contribute terawatts to the global energy budget, engineers will have to invent affordable ways to mine the mother lode. Three high-flying designs are in active development.

Magenn Power in Ottawa, Ontario, plans to begin selling next year a rotating, helium-filled generator that exploits the Magnus effect (best known for giving loft to spinning golf balls) to float on a tether up to 122 meters above the ground. The bus-size device will produce four kilowatts at its ground station and will retail for about $10,000— helium not included. The company aims to produce higher-flying, 1.6-megawatt units, each the size of a football field, by 2010.

"We looked at balloons; the drag they produce seemed unmanageable in high winds," says Al Grenier of Sky WindPower in Ramona, Calif. Gre-nier's venture is instead pursuing autogiros, which catch the wind with helicopterlike rotors. Rising to 10,000 meters, the machines could realize 90 percent of their peak capacity. The inconstancy of surface winds limits ground turbines to about half that. But the company has struggled to gather the $4 million it needs for a 250-kilowatt prototype.

Still in the conceptual stages is the "ladder-mill," designed by astronaut Wubbo J. Ockels and his students at the Delft University of Technology in the Netherlands. Ockels envisions a series of

▲ Autogiros designed by Sky WindPower would use powered counterrotating blades to rise above 10,000 feet, then switch to generating mode. Computers adjust the pitch of the four blades to maintain the craft's position and attitude.

Fast Facts

□ Wind power capacity, currently about 58 gigawatts, is expected to triple by 2014.

□ Helium-filled generators have to be refilled every few months.

□ Number of tethered aerostats monitoring the U.S. border: 8.

REALITY FACTOR

126 meters computer-controlled kites connected by a long tether. The ladder of kites rises and descends, turning a generator on the ground as it yo-yos up and down. Simulations of the system suggest that a single laddermill reaching to the jet stream could produce up to 50 megawatts of energy.

Until high-altitude machines are fielded, no one can be certain how well they will hold up under turbulence, gusts and lightning strikes. Steep maintenance costs could be their downfall.

There are regulatory hurdles to clear as well. Airborne wind farms need less land than their terrestrial counterparts, but their operators must persuade national aviation agencies to restrict aircraft traffic in the vicinity. There is precedent for this, Grenier points out: the U.S. Air Force has for years flown up to a dozen large tethered aerostats at high altitude above the country's southern border.

By the standards of revolutionary technologies, however, high-altitude wind looks relatively straightforward and benign.

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