19 June 2009

Kites could power world 100 times over

By
Cosmos Online
High-flying Kites could harness enough energy to power the world 100 times over, according to a survey of high-altitude winds.
High flying kite

Kite-driven generators have been suggested as one method of capturing this energy. In principle, they work by using the strong pull of the wind to drive a land-based turbine, tethered to the kite via a cable. Credit: Sky Windpower Corporation

SYDNEY: High-flying Kites could harness enough energy to power the world 100 times over, according to a survey of high-altitude winds.

Published in the journal Energies, the study reports that areas well suited to harvesting high altitude winds fall over some of the world’s major cities such as New York and Tokyo.

“The wind energy aloft is phenomenal. Energy densities unthinkable near the ground are common in the upper levels of the atmosphere,” said Cristina Archer, lead author and a meteorologist at California State University in Chico, USA. “It’s like a perpetual source of free energy.”

Fast and furious

“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,” added coauthor Ken Caldeira, a climate scientist at the Carnegie Institution’s Department of Global Ecology in Stanford, California.

Jet streams are moving belts of furious winds, shifting seasonally at altitudes between 6,000 and 15,000 metres. To get a global picture of the energy these jets hold, the researchers compiled 28-years-worth of data from both the U.S. National Centres for Environmental Prediction and the Department of Energy.

Archer and Caldeira looked at both wind speed and air density at different altitudes, concluding that extraordinary amounts of energy exist above Japan, eastern China, the eastern coast of the U.S., southern Australia and north-eastern Africa.

Kite-driven generators

Average wind power densities in these locales “are greater than 10 kilowatts per square metre. This is unthinkable near the ground, where even the best locations have usually less than one kilowatt per square metre,” said Archer. New York clocked up a whopping wind power density of 16 kilowatts per square metre, the study found.

Kite-driven generators have been suggested as one method of capturing this energy. In principle, they work by using the strong pull of the wind to drive a land-based turbine, tethered to the kite via a cable.

When the cable reaches its full extension, the angle of the kite is shifted so that the wind no longer pulls and the cable can be rolled in again, before the cycle repeats. A prototype kite designed by Dutch former astronaut Wubbo Ockels, now at the Delft University of Technology in the Netherlands, was able to generate 10 kilowatts of power – or enough electricity to supply 10 homes.

“This approach has the advantage that the heavy generator stays on the ground” said Pavel Trivailo, an aerospace engineer from the Royal Melbourne Institute of Technology, in Australia. He said he has recently applied for funding to develop Ockels’ technology further, using radio-controlled gliders.

Like kites, gliders would only be useful on their way out into the wind – whilst winding back in, no electricity is generated. Trivailo imagines paired systems of kites, where one travels out as the other returns, likening the system to the paired action of a piston engine.

Gaps in the grid

He points out that cables capable of safely coping with large forces – such as those generated by wind – already exist, thanks to the ‘space tethers’ used to join sections of satellites.

Some challenges of high-altitude wind power are still to be met, however. “While there is enough energy in these high altitude winds to power all of modern civilisation, at any specific location there are still times when the winds do not blow,” said Caldeira.

The study predicts wind may still fail about 5% 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 said.

Trivailo agrees much work needs to be done. “How you deploy the gliders or kites at high altitudes is still an open question,” he said.

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