The researchers set out to find a structure that can both efficiently absorb water and generate steam at the surface of water. After a series of trials, Hadi Ghasemi, a postdoctoral associate in MIT’s Department of Mechanical Engineering, found the perfect answer: a two-layered, disc-shaped sponge.
The top layer is made of graphite, which was placed in a microwave for seven seconds to cause expansion and popping, resulting in flakes. The bottom layer is a carbon foam with pockets of air, enabling it to keep the sponge afloat while acting as a porous insulator.
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When sunlight hits the sponge, hot spots are created in the graphite layer and water is drawn up through the carbon foam. When droplets of water touch the graphite, they’re quickly turned into steam.
“Steam is important for desalination, hygiene systems, and sterilization,” says Ghasemi. “Especially in remote areas where the sun is the only source of energy, if you can generate steam with solar energy, it would be very useful.”
The low-cost sponge is able to convert 85 percent of incoming solar energy into steam, which is far more efficient than traditional alternatives, such as using fields of mirrors or lenses to concentrate sunlight used to heat up large volumes of liquid.
Even with new approaches, such as mixing water with nanoparticles that heat up quickly in sunlight, it takes about 1,000 times the solar energy found on an average sunny day to generate steam. The new approach discovered by the MIT researchers is able to generate steam at about 10 times the solar intensity of an average sunny day with the help of a cheap lens or mirror, the lowest reported optical concentration reported to date.
This new process isn’t without its issues. For one, the pores of the graphite layer get clogged with salt during the desalination process. “We need to figure out how to handle that,” says Gang Chen, a professor at MIT who heads the Department of Mechanical Engineering.
Chen sees a bright future of applications for the wonder sponge, including the generation of steam power and drying up post-flood surfaces.
