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The Sponge That Turns Solar Power Into Steam

Researchers from the Massachusetts Institute of Technology say a new structure capable of converting 85 per cent of solar energy into steam could vastly improve current approaches to solar thermal power generation.
The MIT team created a porous structure of flaked graphite and a layer of carbon sponge which, when floated on water, absorbed and retained solar energy at unprecedented levels. When the graphite layer is exposed to sunlight, it rapidly heats up, creating steam and a pressure gradient which draws yet more water through the carbon layer in a cyclical process.
Previous methods for creating solar steam – outside of massive concentrating solar power (CSP) systems – have mixed high-PV nanoparticles with water to create steam. 
This approach is expensive and requires solar concentrations of around 1,000 times that of an average sunny day. The MIT process only requires a solar intensity about 10 times that of a sunny day – the lowest optical concentration reported so far, the researchers say.
According to Hadi Ghasemi, a postdoc in MIT’s Department of Mechanical Engineering, the spongelike structure can be also made from relatively inexpensive materials.
“Steam is important for desalination, hygiene systems, and sterilization,” says Ghasemi, who led the development of the structure. “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.”
In a report published in the journal Nature Communications, the researchers describe how they used microwaves to bubble the surface of the graphite layer “just like popcorn” to enhance its thermal qualities; then sandwiched it on top of an insulating layer of carbon foam, which serves to keep the structure afloat and allow water to seep up through the layers.
When sunlight hits the graphite, it creates a hot spot that starts the thermal process and begins drawing water up through the structure to create steam. When tested in a solar simulator, the team found the structure converted 85 per cent of solar energy into steam.
Ghasemi believes that with more trials and tweaking materials this figure could be improved and at even lower solar intensities.
“There can be different combinations of materials that can be used in these two layers that can lead to higher efficiencies at lower concentrations. There is still a lot of research that can be done on implementing this in larger systems.”

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