Heat transfer by condensation is key to the operation of today’s power plants, where fossil fuels are used to boil water and the resulting steam drives turbines to generate electricity. The steam must then condense back to water, which is collected and sent back to the boiler to start the cycle again.
The new system is an improvement of the condensers used to turn steam back into water. The same principle might also be used to improve condensers in desalination plants and in thermal-management systems.
Three key qualities contribute to the efficiency of heat transfer in such systems: Droplets must form easily and abundantly on a condenser surface; the area of contact between the droplet and the surface must be large enough to easily conduct heat; and the droplets must quickly fall away from that surface to allow new droplets to start condensing.
While most previous research on improving condensers has focused on the third part, the new work improves all three aspects at once, says associate professor of mechanical engineering Evelyn Wang, senior author of a paper just published in the journal Scientific Reports. The report was co-authored by Rong Xiao and Nenad Miljkovic, both of whom just completed their PhDs at MIT, and former postdoc Ryan Enright.
The innovation combines two properties: First, a nanopatterned surface, etched with tiny pillars, reduces contact between droplets and the surface. Second, a layer of oil coats the surface, helping droplets to form abundantly on the surface and also making it easy for them to slide off.
“We know it’s a combination of these qualities that is optimal,” Wang says. “We believe the big contribution of this work is to drastically enhance [droplet] densities. … We see [droplets] form on every single one of those pillar tops.”
Condensers’ contradictory need to enhance both droplet formation (requiring a hydrophilic surface) and droplet release (requiring a hydrophobic surface) is satisfied by the combination of nanostructures and an oily surface. This results in a surface that is locally hydrophilic — attracting droplets to the tops of the tiny pillars — while still being hydrophobic overall, causing droplets to fall away quickly as they grow in size. The new system produces much greater density of droplets than has been achieved on most other nanopatterned surfaces, Wang says.
Because the droplets condense right through the thin coating of oil, and end up being immersed in oil, the researchers coined the term “immersion condensation” to describe their new system. The new approach can be applied to ordinary copper plates or tubes, typically used in today’s condensers, so it should be relatively easy to incorporate into existing plants, Wang says.
The group initially did computer modeling of the system, then carried out experiments to verify the models’ predictions. The experiments confirmed a 100 percent enhancement of heat-transfer efficiency, compared to untreated copper surfaces.
Further research, using different kinds of oil and different texture patterns, could yield even greater improvements, Wang says. “There’s lots of opportunity for optimization of the structures to get better performance,” she says.
Anthony Jacobi, a professor of mechanical science and engineering at the University of Illinois at Urbana-Champaign, says this research “is very exciting to me, because it not only demonstrates the innovative use of surface heterogeneity to promote immersion condensation, but it carries that idea to a potentially scalable and inexpensive deployment that may be useful in real power-generation or water-recovery systems. … The impact on energy efficiency could be tremendous.”
This work was supported by the Office of Naval Research and MIT’s Solid State Solar Thermal Energy Conversion Center, which is funded by the U.S. Department of Energy.