[ExI] kite power?

[Damien Broderick]

<http://www.cosmosmagazine.com/node/2822/full>


Kites could power world 100 times over

Friday, 19 June 2009

by Kerensa McElroy
Cosmos Online

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.


<http://www.mdpi.com/1996-1073/2/2/307>The study 
in <http://www.mdpi.com/1996-1073/2/2/307>Energies

<http://www.skywindpower.com/ww/index.htm>More on 
high altitude wind power - Sky Windpower Corporation