MIT 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.
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.