MIT Department of Mechanical Engineering researchers have produced a sponge-like composite material that helps convert water to steam by using sunlight only one-hundredth as bright as what conventional steam-producing solar generators require. The new material is a composite of graphite flakes layered on a bed of carbon foam that is reported to convert as much as 85 percent of received solar energy into steam.
The team says the material can be made from relatively cheap materials and may be suitable for a new range of inexpensive, compact, steam-powered applications. Their experiments show far less heat loss than found in ordinary systems and that steam can be generated at much lower temperatures, so future solar-to-steam systems may be cheaper and less complex to build and maintain.
The composite forms a porous insulating material structure that floats on water. The scientists expanded the graphite by cooking it in a microwave, giving it a permeable top layer that can absorb and retain more solar energy. The bottom layer is carbon foam with hundreds of tiny air pockets that keep the material floating on the water and provide insulation. The key property that allows steam generation is that the foam is also riddled with tiny pores that allow water to move through the material through capillary action from applied heat. When sunlight 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, which then converts the water into steam.
“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.”