Power from Low-Level Sunlight

Wednesday, July 30, 2014 @ 11:07 AM gHale

A sponge-like substance that helps convert water to steam using sunlight not as bright as that required by conventional steam-producing solar generators is now in development.

Generating the steam is possible by creating a composite of graphite flakes layered on a bed of carbon foam and this new material can convert as much as 85 percent of received solar energy into steam, said researchers at MIT’s Department of Mechanical Engineering.

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In practice, the researchers said the graphite flakes and carbon foam composite they created forms a porous insulating material structure that floats on water. After a number of experiments, the researchers found 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 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 also has tiny pores that allow water – through capillary action from applied heat – to make its way up through the material.

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 draws up through the material, and the more steam generated.

“Steam is important for desalination, hygiene systems, and sterilization,” said 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 researchers said the sponge-like material consists of relatively 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 material and experiment shows there is much less heat loss than that found in ordinary systems and steam could end up generated at much lower temperatures, future solar-to-steam systems may be much cheaper and less complex to build, run, and maintain.



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