At MIT, droplets dropping faster prove efficiency for powerplants

The condensation of water is crucial to the operation of most powerplants–whether they are fueled by coal, natural gas or nuclear fuel.

A study by a team at MIT offers new insights to how droplets form when water condenses on a surface, and how to pattern the collecting surfaces at a nanoscale in order to encourage droplets to form more rapidly. These insights could enable a significantly more efficient power plants and desalination plants, the researchers said.

Typically, on a condensing surface, droplets gradually grow larger while sticking to the material through surface tension. Once they get so big that they drop, they rain down into a container below. But it turns out out there are ways to get them to fall–even jump–from the surface at much smaller size, and long before gravity takes over. This reduces the size of the removed droplets and makes the resulting transfer of heat much more efficient.

Few researchers have looked at the growth of the droplets prior to the jumping in detail. This is important because if the jumping-effect allows droplets to leave the surface faster than they would otherwise, and if their growth lags, you might actually reduce efficiency. It’s not just the size of the droplet when it gets released that matters, but also how fast it grows to that size.

“This has not been identified before,” Nenad Miljkovic, a mechanical engineering graduate student, said about the project.”You think you are getting enhanced heat transfer, but you’re actually getting worse heat transfer.”

The enhanced efficiency could improve the rate of water production in plants that produce drinking water form seawater, or even in proposed new solar-power systems that rely on maximizing evaporator (solar collector) surface area and minimizing condenser (heat exchanger) surface area to increase the overall efficiency of solar-energy collection.

The next step in the research is to extend the findings from the droplet experiments and computer modeling– and find even more efficient configurations and ways of manufacturing them rapidly and inexpensively on an industrial scale, Miljkovic said.