MIT is reporting that it has created a new, cheap material — using a microwave, no less — that converts sunlight into steam with an amazing 85% efficiency. This could have major repercussions in the realms of desalination and sterilization, and perhaps for concentrated solar thermal power generation as well.
The new material, developed by MIT mechanical engineer Hadi Ghasemi, consists of a thin double-layered disc. The bottom layer consists of spongy carbon foam that doubles up as a flotation device and a thermal insulator that prevents solar energy from dissipating into the fluid underneath. The top layer — the active layer — consists of flakes of graphite that were exfoliated using a microwave. The microwave causes the graphite to bubble up “just like popcorn” according to Gang Chen, another researcher involved with the work. [doi:10.1038/ncomms5449 – "Solar steam generation by heat localization"]
MIT’s spongy graphite-carbon solar steam machine — diagram on the left, in action on the right
When sunlight hits the graphite, hot spots are created that draw water up through the carbon foam via capillary action. When the water reaches the hot spots in the graphite, there’s enough heat to turn the water into steam. The efficiency of the material is linked to the amount of incoming light — at a solar concentration (intensity) of 10 times that of a typical sunny day, 85% of incoming solar energy is converted into steam (assuming there’s enough water nearby; this doesn’t magically create steam out of thin air). “There can be different combinations of materials that can be used in these two layers that can lead to higher efficiencies at lower concentrations,” says Ghasemi. Graphene, anyone?
As for what this little spongy steam-maker might actually be used for, there’s a variety of possibilities. The low solar intensity requirement (10x is easy to obtain with a simple lens or reflector) means this could a very good way of producing clean water or sterilizing equipment (to this day, steam is still a very popular way of sterilizing things). Bulk desalination is another possibility, though we wonder if the carbon foam wouldn’t get clogged up with the leftover salt crystals.
The graphite-carbon sponge, up close
And then there’s the most exciting possibility: Good ol’ power generation. In modern-day concentrated solar thermal power generation, fresnel lenses or parabolic reflectors are used to concentrate sunlight by up to 1,000 times. If steam can be produced with just the intensity of 10 suns, then system costs can probably be reduced and overall efficiency increased. A lot more work needs to be done before this stuff revolutionizes power generation, though: So far, though, MIT hasn’t gone any further than “ooh, this stuff produces steam!” As we mentioned before with regards to desalination, it’s very likely that this new material would clog up with mineral deposits rather quickly (i.e. fouling), completely destroying any semblance of efficiency.
Still, it’s clearly early days. Problems like fouling (limescale! corrosion!) have been around forever, and as such there are lots of ways to combat it. If MIT really has stumbled across a way of cheaply and easily producing steam from sunlight, then this could be big news.
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