Solar sponge: The top layer of graphite soaks up the sun's energy in tiny holes. When drops of liquid fill the holes, the water quickly evaporates. (The beaker looks hot, but the water below the sponge is cool as a cucumber.)
Piscine Molitor "Pi" Patel did it to survive on the Pacific Ocean. Robert Redford used the trick in All Is Lost.
When you're trapped on a boat, you can easily make fresh water, right? Simply let the sun heat up and evaporate salt water. Then trap the steam, condense it on a plastic surface and collect the fresh water. The liquid even gets sterilized in the process.
So why can't people around the world who lack clean drinking water do something similar?
Turns out, desalinating or sterilizing water with solar energy is way harder than Hollywood makes it look. The process is super inefficient and way too slow to be practical.
"The average yield is only about 1 cup per day," says the U.S. Air Force survival guide, even when you've got eight hours of sun and plenty of water.
But engineer Hadi Ghasemi, at the University of Houston, is trying to change that. He and a team at the Massachusetts Institute of Technology have developed a cheap material that desalinates water efficiently and fast using solar energy. And the secret to the new technology was sitting right on their desks: the graphite in pencils.
A simple solar still — and even more expensive versions with mirrors and lenses — heats up the entire water surface before it starts to evaporate, Ghasemi says. That takes time and wastes energy.
Courtesy of the researchers/MIT
"Why do we need to heat the bulk of the liquid to get steam?" Ghasemi says. "Why not concentrate the solar energy at 'hot spots?' " Then all the energy goes into creating steam.
The trick to creating these "hot spots" is having the right material, he says. And that's where the graphite in pencils comes into play.
"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. "So you don't need such expensive optical systems to concentrate the solar energy."
Ghasemi and his team still have many kinks to iron out before they can turn the technology into a useful product. A major one is what to do with all the salt.
"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."
Although the material is highly efficient at converting solar energy into steam, the material still requires a cheap lens or mirror to concentrate sunlight by about tenfold. (By comparison, other technologies require 1,000-fold concentration of the light, which requires expensive optics.)
"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."
Still, though, Chen is excited about developing the sun sponge for a myriad of other applications, such as making steam power and drying up surfaces after floods.
"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."
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