Sponge-like structure generates steam using lowest concentration of solar energy yet
Researchers working at MIT’s Department of Mechanical Engineering claim to have produced a sponge-like substance that helps convert water to steam using sunlight one-hundredth as bright as that required by conventional steam-producing solar generators. A composite of graphite flakes layered on a bed of carbon foam, the new material is reported to convert as much as 85 percent of received solar energy into steam.
In practice, the scientists say that the graphite flakes and carbon foam composite that they've created forms a porous insulating material structure that floats on water. After a number of experiments, the scientists found that the best method to maximize heat retention properties in the top layer was to exfoliate (expand a material by heating so that it increases in volume and lowers in density) graphite by cooking it in a microwave, causing it to bubble and swell. The outcome is an exceedingly permeable top layer able to maximize absorption and retention of solar energy.
The bottom layer is fashioned from carbon foam containing hundreds of tiny pockets of air that keeps the material floating on the surface of the water, while also providing insulation that prevents heat escaping to the water underneath it. Most importantly for the generation of steam, the foam is also riddled with tiny pores that allow water – through capillary action from applied heat – to make its way up through the material.
As such, when sunlight illuminates and heats the material it generates a pressure differential between the foam and the air that draws water up through the carbon and into the graphite layer. As the water soaks into the graphite, the heat focused on the material then converts the water into steam. As a result, the more intense the light striking the graphite surface, the more water is drawn up through the material, and the more steam generated.
"Steam is important for desalination, hygiene systems, and sterilization," says Hadi Ghasemi, a postdoctoral MIT student who ran the material development. "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 team also claims that the sponge-like material is able to be made from comparatively cheap materials and may be suitable for a new range of inexpensive, compact, steam-powered applications, particularly as this method offers a significant improvement over conventional solar-powered steam generation methods.
Though not in the same league as supercritical solar steam generators, the upshot of the demonstrated material and experiment shows that there is much less heat loss than that found in ordinary systems and that as steam may be generated at much lower temperatures, future solar-to-steam systems may be much cheaper and less complex to build, run, and maintain.
The research was recently published in the journal Nature Communications.