Harnessing Direct Solar Power for Propulsion

Technology.am (Apr. 13, 2009) — All the technologies that capitalize on sunlight, including photovoltaics and biofuels, require intermediate infrastructure to turn the sun’s rays into something that can be used to perform work in a machine.


Solar spin: A four-finned rotor floating on a pool of water spins when exposed to sunlight. At left is a lens used to direct sunlight onto the rotor; the bright shape next to the rotor is reflected sunlight.

Researchers at the University of California, Berkeley, are applying carbon nanotubes to build small, simple waterborne machines propelled directly by sunlight. Theoretically, they say, these machines could be scaled up to make energy-generating pumps directly powered by the sun.

The sun-powered machines rely on water’s surface tension. Water molecules are strongly attracted to one another. These high-energy interactions can, under the right conditions, pull objects across the water.

The Berkeley machines are pieces of clear plastic, about a centimeter on their longest edge, embedded with strips of vertically aligned carbon nanotubes. When light from the sun or from a laser is focused on the machine floating on a pool of water, the nanotubes heat up and heat water around them. This causes a decrease in surface tension localized to one region of the machine, which is in turn propelled forward away from the low-tension part of the surface.

Other similar systems break surface tension using electrical pulses, but this requires a power source such as a battery or a solar cell. “This is better because you eliminate the middleman and gets a lot of work out,” says Alex Zettl, a professor of condensed-matter physics at Berkeley.

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