Sustainable Energy

Harnessing Direct Solar Power for Propulsion

Tiny boats made of nanomaterials are powered directly by sunlight.

The sun is the most abundant source of renewable energy. But all the technologies that capitalize on sunlight, including photovoltaics and biofuels, require intermediate steps and infrastructure to turn the sun’s rays into something that can be used to perform work in a machine. Researchers at the University of California, Berkeley, are using carbon nanotubes to build small, simple waterborne machines propelled directly by sunlight. In theory, they say, these machines could be scaled up to make energy-generating pumps directly powered by the sun.

Solar spin: A four-finned rotor (center) 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.

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 the 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 get a lot of work out,” says Alex Zettl, a professor of condensed-matter physics at Berkeley who led the research team with Jean M.J. Fréchet, a professor of chemistry and chemical engineering. “We think we’re on to something because surface tension is very powerful,” says Zettl.

So far, the Berkeley team has demonstrated two basic sun-powered machines. The first, a plastic rectangular boat with a nanotube strip at its back, performs linear motion. By directing laser light or using a lens to focus sunlight either at the center of the nanotube strip or at its corners, the boat can be directed straight forward or in circles. When light is focused onto it, a boat about a centimeter long can travel as fast as eight centimeters per second. The second machine is a simple rotor with one nanotube strip on one side of each of its four fins. When exposed to direct sunlight, it spins at about 70 rotations per minute. Both machines have only been tested in containers in the lab.

Carbon black: Vertical arrays of carbon nanotubes, shown here embedded in a clear polymer, absorb nearly all the sunlight that hits them. This scanning electron micrograph shows the edge of one of the boats.

“This is pretty simple stuff, but it’s made possible by sophisticated materials,” says Zettl. Earlier this month, researchers at Japan’s Meijo University established that carbon nanotubes arranged in forestlike, vertical arrays are the blackest materials ever tested, absorbing almost all the light that falls on them. “This material has the ideal properties for collecting energy from the sun,” says Fréchet.

Zettl and Fréchet say that, in theory, these thermal surface-tension effects should be scalable. The Berkeley group started with millimeter-scale machines not only because they were convenient to test in the lab, but also because manipulating objects this size in liquid poses particular challenges. Turbulence is a huge factor at the millimeter scale, says Fréchet. The light-powered mechanism could potentially be used to move nanoscale objects through microfluidic devices employed for medical diagnostics. At the nanoscale, says Fréchet, “surface tension beats gravity.” The researchers also hope to scale up their work to make actual boats. Lenses mounted on the back of a large boat should focus sufficient sunlight onto the absorbent nanotubes to propel it. They also hope to make large nanotube-embedded rotors for generators powered by the sun.

“Now they need to see if this will operate in a real environment,” says Dean Alhorn, lead engineer on NASA’s solar-driven satellite NanoSail-D. NASA’s satellite, which was tested this summer, uses a reflective material to absorb the momentum, rather than the heat, from photons; this technology should work well in the vacuum of space but hasn’t been practical on Earth. Alhorn says that Zettl and Fréchet have solidly demonstrated that it’s the light, not something else, causing the tiny boats to move. However, he notes that the machines have only been tested in tubs of water inside the lab. Alhorn says that the researchers will have to answer the question, “How much force can they generate, versus opposing forces like waves in the real world?”

Indeed, the Berkeley researchers say that their next step is further engineering their devices. “This could be very efficient because nanotubes absorb light so well, but we have to see if this is a viable thermodynamic system,” says Zettl.

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