A four-finned rotor (center) floating on a pool of water spins when exposed to 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.