(Newswire.net — September 6, 2014) — Turns out, desalinating or sterilizing water with solar energy is way harder than Hollywood makes it look (Piscine Molitor “Pi”, Robert Redford in “All Is lost”).
In theory, all you need is to desalinate sea water by simply let the sun heat up and evaporate salt water. Then trap the steam, condense it on a plastic surface and collect the fresh sterilized water.
In practice, the process is super inefficient and way too slow. You need all day for processing only one cup of fresh water that way.
But engineer Hadi Ghasemi, at the University of Houston, is trying to change that. He and a team at the Massachusetts Institute of Technology (MIT) have developed a cheap material that desalinates water efficiently and fast using solar energy.
The trick is not to heat whole surface but only hot spot, and the secret to creating these “hot spots” is having the right material, says Ghaseni.
“We took graphite and put it into the microwave for seven seconds,” Ghasemi says. The gases in the mineral cause the outer layer to expand and pop. “It’s exactly like a popcorn!”
The result is a thin, porous material that looks like a black sponge. It floats on the surface of water, like a sponge, but instead of soaking up liquid, the pores soak up the sun, Ghasemi and his colleagues reported in the journal Nature Communications back in July.
The graphite has holes in it with just the right shape to concentrate solar energy and create tiny hot spots in the graphite. Water creeps into the holes through capillary action (just as water moves up the stem of a plant to its leaves). The droplets then heat up quickly and evaporate.
“It creates steam at a low concentration of solar energy,” Ghasemi says.
“When water desalinates, it leaves behind the salt. Eventually the pores [of the graphite] will be clogged,” says Gang Chen of MIT, who led the study. “We need to figure out how to handle that.”
Graphite is highly efficient at converting solar energy into steam, however, the material still requires a cheap lens or mirror to concentrate sunlight by about tenfold.
“We want to further reduce the concentration of sunlight needed,” Chen says. “Then the technology wouldn’t need fancy tracking technology to keep the sun focused on it.”
“The raw materials are very cheap compared to those used in other solar power generation now,” he says. “The idea is just so simple. I don’t know why we didn’t think about it earlier.” says Chen.