A “solar steam” sterilization system that uses light-harvesting nanoparticles to convert as much as 80 percent of sunlight energy into germ-killing heat could be a boon for more than 2.5 billion people who lack adequate sanitation. “Sanitation and sterilization are enormous obstacles without reliable electricity,” said Naomi Halas, director of Rice University’s Laboratory for Nanophotonics (LANP); the Stanley C. Moore Professor in Electrical and Computer Engineering; and a professor of biomedical engineering, chemistry, physics and astronomy. “Solar steam’s efficiency at converting sunlight directly into steam opens up new possibilities for off-grid sterilization that simply aren’t available today.” Halas and colleagues demonstrated two ways that solar steam can be used for sterilization — one setup to clean medical instruments and another to sanitize human waste. The setups were reported in the Proceedings of the National Academy of Sciences Early Edition (doi: 10.1073/pnas.1310131110). In a study last year, Halas’ team showed that solar steam was so effective at direct conversion of solar energy into heat that it could even produce steam from ice water. (See: Photonic Nanocrystals Convert Sunlight to Solar Steam) “It makes steam directly from sunlight,” she said. “That means the steam forms immediately, even before the water boils.” Rice University graduate student Oara Neumann, left, and scientist Naomi Halas are co-authors of a new study about a highly efficient method of turning sunlight into heat. They expect their technology to have an initial impact as an ultrasmall-scale system to treat human waste in developing nations without sewer systems or electricity. Courtesy of Jeff Fitlow/Rice University. The solar-powered sterilization system’s efficiency comes from light-harvesting nanoparticles developed at LANP by Rice graduate student Oara Neumann. Neumann created a version of nanoshells that converts a broad spectrum of sunlight — including both visible and invisible bandwidths — directly into heat. When submerged in water and exposed to sunlight, the particles heat up so quickly they instantly vaporize water and create steam. The technology has an overall energy efficiency of 24 percent. Photovoltaic solar panels, by comparison, typically have an efficiency of around 15 percent. When tested in autoclaves — designed by Rice undergraduates at Rice’s Oshman Engineering Design Kitchen and refined by Neumann and colleagues at LANP — the heat and pressure created by the steam were sufficient to kill not only living microbes, but spores and viruses also. Standard tests for sterilization showed the solar steam autoclave could kill even the most heat-resistant microbes. “The process is very efficient,” Neumann said. “For the Bill & Melinda Gates Foundation program that is sponsoring us, we needed to create a system that could handle the waste of a family of four with just two treatments per week, and the autoclave setup we reported in this paper can do that.” The team hopes to work with waste-treatment pioneer Sanivation to conduct the first field tests of their waste sterilizer at three sites in Kenya. “Sanitation technology isn’t glamorous, but it’s a matter of life and death for 2.5 billion people,” Halas said. “For this to really work, you need a technology that can be completely off-grid, that’s not that large, that functions relatively quickly, is easy to handle and doesn’t have dangerous components. Our Solar Steam system has all of that, and it’s the only technology we’ve seen that can completely sterilize waste. I can’t wait to see how it performs in the field.” The research was supported by a Grand Challenges grant from the Bill & Melinda Gates Foundation and by the Welch Foundation. For more information, visit: www.rice.edu