A new sterilization system uses nanomaterials to convert 80 percent of the energy in sunlight into heat, and could be a boon for more than 2.5 billion people who lack adequate sanitation.
The technology, described online in the early edition of Proceedings of the National Academy of Sciences, offers two ways that solar steam can be used for sterilization—one setup to clean medical instruments and another to sanitize human waste.
“Sanitation and sterilization are enormous obstacles without reliable electricity,” says Naomi Halas, director of the Laboratory for Nanophotonics (LANP) at Rice University. “Solar steam’s efficiency at converting sunlight directly into steam opens up new possibilities for off-grid sterilization that simply aren’t available today.”
In a previous study last year, Halas and colleagues showed that “solar steam” was so effective at direct conversion of solar energy into heat that it could even produce steam from ice water.
“It makes steam directly from sunlight,” she says. “That means the steam forms immediately, even before the water boils.”
Solar steam’s efficiency comes from light-harvesting nanoparticles that were created at LANP by Rice graduate student Oara Neumann, the lead author of the PNAS study.
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 overall energy efficiency of around 15 percent.
When used in the autoclaves in the tests, the heat and pressure created by the steam were sufficient to kill not just living microbes but also spores and viruses. In the study, standard tests for sterilization showed the solar steam autoclave could kill even the most heat-resistant microbes.
“The process is very efficient,” Neumann says. “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 researchers hope to conduct the first field tests of the solar steam 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 says. “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 study was supported by the Bill & Melinda Gates Foundation and by the Welch Foundation.