Saturday, 25 October 2014

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The big turnoff

John McKenna, Editor

MIT research could help improve plant efficiency

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Massachusetts, US- Research by a team at the Massachusetts Institute of Technology (MIT) has provided insights into how condensation forms on a surface.

According to the group, the results could significantly increase the efficiency of the next generation of power and desalination plants.

Typically, on a condensing surface, droplets grow larger while adhering to the material through surface tension. When they are large enough, gravity overcomes the surface tension and droplets rain down into a container.

MIT mechanical engineering graduate student, Nenad Miljkovic explained that there are ways to make droplets fall from a surface at smaller sizes, enabling the resulting transfer of heat to be much more efficient.

One mechanism is a surface pattern that encourages adjacent droplets to merge together. As they do so, energy is released, which causes a recoil from the surface.

By incorporating measurements of droplet growth rates and heat transfer into computer models, the MIT team was able to compare a variety of approaches to developing a surface pattern.

The researchers found one promising option was to create a forest of pillars on a surface at nanoscale. They reported that droplets tended to sit on top of the pillars while only locally wetting the surface, minimising the area of contact and facilitating easier release.

“We showed that our surfaces improved heat transfer by up to 71% [compared to flat, non-wetting surfaces currently used only in high-efficiency condenser systems] if you tailor them properly,” said Miljkovic.

The enhanced efficiency could improve the rate of water production in plants that produce drinking water from seawater, or in solar-power systems.

A similar system could improve heat removal in computer chips, which is often based on internal evaporation and recondensation of a heat-transfer liquid through a device called a heat pipe.

MIT is now extending its work to find ways of manufacturing these surfaces rapidly and cheaply on an industrial scale.

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