Sponging Up Solar Energy

Leave it to those smarties at MIT to come up with something that sounds more like science fiction than reality: a new “material structure” that generates steam by soaking up the sun's rays.

As reported last week by Science Daily, this sponge-like structure is a layer of graphite flakes and an underlying carbon foam, which all works to create a “porous, insulating material structure that floats on water. When sunlight hits the structure's surface, it creates a hotspot in the graphite, drawing water up through the material's pores, where it evaporates as steam. The brighter the light, the more steam is generated.”

Could this mean a return to the age of steam? Let’s not get too far ahead of ourselves. "Steam is important for desalination, hygiene systems and sterilization," Hadi Ghasemi, who led the development of the structure, says in the article. Ghasemi is a postdoc in MIT’s Department of Mechanical Engineering. "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," he adds.

Ghasemi and mechanical engineering department head Gang Chen, along with five others at MIT, provided on the details of the new steam-generating structure in the journal Nature Communications, in a report entitled Solar steam generation by heat localization.

The journal abstract says that steam generation using solar energy currently “is based on heating bulk liquid to high temperatures” by using huge fields of mirrors or lenses that concentrate the incoming sunlight. This approach requires “either costly high optical concentrations leading to heat loss by the hot bulk liquid and heated surfaces or vacuum.”

New solar receiver concepts such as porous volumetric receivers or nanofluids have been proposed to decrease these heat losses. But initiating these reactions requires very intense solar energy -- about 1,000 times that of an average sunny day.

Thus the new structure developed by the MIT research team “provides a novel approach to harvesting solar energy for a broad range of phase-change applications,” the abstract says. It generates steam at a solar intensity of about 10 times that of a sunny day, which is the lowest optical concentration reported thus far, the Science Daily article says. The implication, the researchers say, is that steam-generating applications can function with lower sunlight concentration and less-expensive tracking systems.

"This is a huge advantage in cost-reduction," Ghasemi says. "That's exciting for us because we've come up with a new approach to solar steam generation."

After trying multiple materials, he settled on a thin, double-layered, disc-shaped structure. Its top layer is made from graphite that the researchers exfoliated by placing the material in a microwave. The effect, Chen says, is "just like popcorn": The graphite bubbles up, forming a nest of flakes. The result is a highly porous material that can better absorb and retain solar energy.

Sponges! Microwaves! Popcorn! Foam! Flakes! Exfoliation! Solar steampunk technology shines on.

Image credit: From the MIT report, courtesy of the researchers.