September 2008

Process steam: A two-minute egg in 12 seconds

Process and thermal power plants have big boilers that burn fuel to make heat and steam. Millions of dollars of a plant's budget can go to paying for such fuel.

Would engineers and plant managers being interested in cutting that expense by 90%?

A new study from researchers at Rensselaer Polytechnic Institute (RPI) shows by adding an invisible layer of the nanomaterials to the bottom of a metal vessel, an order of magnitude increase in efficiency is possible in bringing water to boil.

This increase in efficiency could have a big impact on reducing costs for industrial boiling applications, cooling computer chips, and improving heat transfer systems.

Bringing water to a boil, and the related phase change that transforms the liquid into vapor, requires an interface between the water and air.

In a pot of water, two such interfaces exist-at the top where the water meets air and at the bottom where the water meets tiny pockets of air trapped in the microscale texture and imperfections on the surface of the pot.

Even though most of the water inside of the pot has reached 100°C and is at boiling temperature, it cannot boil because it is surrounded by other water molecules and there is no interface (no air) present to facilitate a phase change.

The RPI team found by depositing a layer of copper nanorods on the surface of a copper vessel, the nanoscale pockets of air trapped within the forest of nanorods "feed" nanobubbles into the microscale cavities of the vessel surface and help to prevent them from getting flooded with water. This synergistic coupling effect promotes robust boiling and stable bubble nucleation, with large numbers of tiny, frequently occurring bubbles.

"We observed a 30-fold increase in active bubble nucleation site density-a fancy term for the number of bubbles created-on the surface treated with copper nanotubes over the nontreated surface," said Nikhil Koratkar, project leader and RPI professor. "If the time taken to boil a given quantity of water is reduced by an order of magnitude, that should translate into significant cost savings."