Clean Power

Published on August 4th, 2014 | by Sandy Dechert


MIT Turns Solar Steam Into Cheaper Energy, Clean Water

August 4th, 2014 by  

Properties of graphite popcorn systemMIT scientists and engineers have a new twist on phase-changing renewable technology. It combines the most efficient generation of solar steam to date by our favorite energy machine, the sun, and a new graphite-based collection system.

This method puts out the lowest optical concentration reported thus far: steam generation at an intensity about 10 times that of a sunny day. The new material will enable steam generators to function with much lower sunlight concentration and cheaper tracking systems. It has the potential to replace both the super-high-intensity nanoparticle generation method and the relatively inefficient and massive mirror fields previously used to produce steam.

The secret is a material that can both efficiently absorb sunlight and generate steam at a liquid’s surface. It is hydrophilic and thermally insulating, absorbs in the solar spectrum, and has interconnected pores. Hadi Ghasemi, a postdoc in MIT’s Department of Mechanical Engineering, led the development of the structure. He and and mechanical engineering head Gang Chen, along with five others at MIT, report on the details of the new steam-generator in the journal Nature Communications.

Basically, the solar steam process starts by exfoliating a thin layer of graphite. You place it in a microwave and create graphite “popcorn.” The graphite bubbles up and forms a nest of flakes, resulting in a porous material that absorbs and retains solar energy more effectively than current methods.

Researchers then array the material on a thin, double-layered, disc-shaped structure, graphite on top. On the bottom is a sponge-like carbon foam containing pockets of air that both keep the foam afloat and insulate the underlying liquid. This foam has very small pores to allow water to float up through the structure via capillary action.

Cross-section of representative solar-to-steam structure (MIT)Enter the sunlight. It hits the structure and creates a hotspot in the graphite layer. The pressure gradient  generated draws water up through the carbon foam. As water seeps into the graphite layer, the heat concentrated in the graphite turns the water into steam.

In principle, it’s just like a sponge placed in water on a hot, sunny day, which can continuously absorb and evaporate liquid. Along with its potential power uses, the solar steam system will be able to desalinate and/or decontaminate impure and waste water. One commenter suggested that one day, it may even enable solar-powered hot water cars! Scaled up, this arrangement of relatively inexpensive materials would likely eliminate the need for complex, costly systems to concentrate sunlight. The research team was able to convert 85% of solar energy into steam at an intensity 10 times that of a typical sunny day. Ghasemi says different combinations of materials used in the two layers may lead to even higher efficiencies at lower concentrations. More work for MIT, and greater glory.

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About the Author

covers environmental, health, renewable and conventional energy, and climate change news. She's currently on the climate beat for Important Media, having attended last year's COP20 in Lima Peru. Sandy has also worked for groundbreaking environmental consultants and a Fortune 100 health care firm. She writes for several weblogs and attributes her modest success to an "indelible habit of poking around to satisfy my own curiosity."

  • NRG4All

    It seems to me there is another problem besides redundancy. The steam here is at one atmosphere. In order to do any useful work the steam needs to be at a much higher pressure. But if you contain the water the boiling point goes up. I don’t see how this can be useful without some sort of a sun concentrator.

    • Blacksheep

      Well, It needs a sun concentrator anyways at 10 times concentration.

      But you’re dead on though, I too was wondering about the pressure of generation. I had hoped that they were talking about generating equivalent pressures of steam as the concentrated solar power towers they are comparing them to.

  • Sandy Dechert

    Apologies to Jo! We must have crossed a wire here somewhere….

  • GCO

    one day, it may even enable solar-powered hot water cars!

    Er… how exactly?

    • Offgridman

      As it said that was just from one of the commenter’s, not an actual application proposal.
      But if the concentrating effects of this get good enough you could have solar powered steam running a low heat electric generator (like being developed to make better use of the steam from Geothermal), to power the electric motors in the wheels. Or even use the steam to direct drive the wheels as was used in trains for quite a while and some early automobiles.
      Granted it is just a pipe dream for now, as scales of efficiency have a long way to go. But we never get any progress without some dreaming. If it does happen it would most likely be in small lightweight personal run abouts, not full size automobiles like you are thinking. But the low temperature generation of this technique combined with the recapture and reuse systems being developed by the Geothermal processes could make it feasible someday. And notice that is feasible, not necessarily practical.

  • JamesWimberley

    Jo Borras covered this here already on July 23. It’s interesting, but not that interesting!

    • Sandy Dechert

      Apologies to Jo and all! We must have crossed a wire here somewhere….

    • Matt

      LOL, for me that is the first related link.

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