Better surfaces could improve heat transfer to advance the design of concentrated solar-power systems

June 26, 2012 // By Paul Buckley
Researchers at MIT have found that relatively simple, microscale roughening of a surface can enhance its transfer of heat which could play an important part in many developing technologies, including advanced microchips and concentrated solar-power systems.

Such an approach could be far less complex and more durable than approaches that enhance heat transfer through smaller patterning in the nanometer range. The new research also provides a theoretical framework for analyzing the behavior of such systems, pointing the way to even greater improvements.

The work was published this month in the journal Applied Physics Letters, in a paper co-authored by graduate student Kuang-Han Chu, postdoc Ryan Enright and Evelyn Wang, an associate professor of mechanical engineering.

“Heat dissipation is a major problem” in many fields, especially electronics, Wang said; the use of phase-change liquids such as boiling water to transfer heat away from a surface “has been an area of significant interest for many decades.” But until now, there has not been a good understanding of parameters that determine how different materials  - and especially surface texturing - might affect heat-transfer performance. “Because of the complexities of the phase-change process, it’s only recently that we have an ability to manipulate” surfaces to optimize the process, Wang says, thanks to advances in micro- and nanotechnology.

Chu said a major potential application is in server farms, where the need to keep many processors cool contributes significantly to energy costs. While this research analyzed the use of water for cooling, he added that the team “believes this research is generalizable, no matter what the fluid.”

The team concluded that the reason surface roughness greatly enhances heat transfer - more than doubling the maximum heat dissipation - is that it enhances capillary action at the surface, helping keep a line of vapor bubbles ‘pinned’ to the heat transfer surface, delaying the formation of a vapor layer that greatly reduces cooling.

To test the process, the researchers made a series of postage-stamp-sized silicon wafers with varying degrees of surface roughness, including some perfectly smooth samples for comparison. The degree of roughness is measured as the portion of the surface area that can