Generating steam from sun light

On the left, a representative structure for localization of heat; the cross section of structure and temperature distribution. On the right, a photo of enhanced steam generation by the structure under solar illumination. Credit: Hadi Ghasemi et al.

Sun light is harvested by photovoltaic plants and it is also used to heat up water and other materials (like sodium) to generate electricity.  The photovoltaic conversion of light energy into electrical energy has improved but it is still low (in the best condition as of September 2013 the maximum efficiency has reached 44.7% in the lab, and using quite expensive technologies). Conversion of sun light energy into heat and then into electricity does not have higher conversion efficiency (a very good steam turbine has a conversion efficiency of 40% and then you need to factor in the sun light to heat conversion efficiency...) but the advantage is that you can have a thermal storage of energy that is not possible with photovoltaic. In this latter the electricity produced by sun light has to be sent to the grid immediately, whilst in the case of thermal conversion one can keep the heat in thermal storage (often based on sodium salt) and use th wheat to power a turbine when electricity is needed.

The problem with thermal conversion is that you need to have plants with thousands of mirrors to concentrate sun light on the thermal storage to heat it up, and this cost quite a bit. With present systems you need to intensify sun light 1,000 times to generate steam from water (the figure refers to an average sunny day in a temperate region).

Now at MIT researchers have found a way to use a special carbon sponge to create steam with a very high efficiency, over 85%, with much less need for concentrator mirrors. In fact the carbon sponge lowers the concentration need to 10 times (of an average sunny day). This dramatically lowers the cost.

Steam is generated at the surface of the water (of any liquid, and that includes the surface of the water that touches the bottom of the pot) and so researchers looked for a material that can absorb light efficiently and can show a broad surface. They eventually settled for graphite cooked in a microwave to create a sort of popcorn like structure, a highly porous material that can absorb sun light very well. This graphite disk is layered on a carbon foam that containing air bubbles is light and float in the water working also as an insulator keeping the heat in the liquid underneath. Water can percolate through the porous sponge through capillary action.

When the sun light hits it heats up the graphite disc generating a pressure gradient that pulls up the water through the carbon sponge. As it reaches the graphite disc it absorb the heat and converts into steam. The physics behind this is similar to the one you see at work on a beach: a sponge can dry up pretty quickly by moving the water through capillary action and dispersing the vapour in the air.

The application of this technology may be of interest particularly in developing Countries where the produced steam can be used to produce distilled water (creating drinkable water by de-salinisation), hygiene systems and sterilisation. 

Author - Roberto Saracco

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