By David L. Chandler, MIT News Office

Water-Shedding Surfaces Can be Made to Last
On typical hydrophobic coatings, droplets forming from high-temperature steam spread out to coat the surface, quickly degrading their performance. The new coating, seen here, maintains its ability to foster droplet formation over long periods.

Image courtesy of the researchers

Steam condensation is key to the worldwide production of electricity and clean water. It is part of the power cycle that drives 85 percent of all electricity-generating plants and about half of all desalination plants globally, according to the United Nations and International Energy Agency. MIT researchers say they have found a way to improve the efficiency of this process, which could have an enormous impact on global energy use.

It has been known for years that making steam-condenser surfaces hydrophobic — getting them to repel water — could improve the efficiency of condensation by causing the water to quickly form droplets. But most hydrophobic materials have limited durability, especially in steamy industrial settings. The new approach to coating condenser surfaces should overcome that problem, the MIT researchers say.

The findings were published in the journal Advanced Materials by MIT professors Karen Gleason and Kripa Varanasi, graduate student Adam Paxson and postdoc Jose Yagüe.

The solution begins with the discovery of a way to make highly durable polymer coatings on metal surfaces. The covalent-bonding process the team developed is significantly more stable than previous coatings, even under harsh conditions, said Varanasi, the Doherty Associate Professor of Mechanical Engineering.

Tests of metal surfaces coated using the team's process show a stark difference. In tests, the material stood up well even when exposed to steam at 100°C [212°F] in an accelerated endurance test. Typically, the steam in power-plant condensers would only be about 40°C [104°F], Varanasi says.

When the materials that are currently used to make surfaces hydrophobic are exposed to 100°C steam, they begin to degrade after about one minute. The condensing water becomes a film that kills the hydrophobic surface and degrades the heat transfer by a factor of seven, according to Paxson. By contrast, the new material shows no change in performance after prolonged endurance tests. According to degradation models, the material might be durable for much longer than these initial tests, as much as 10 years, according to researchers.

Varanasi and Paxson were part of a team that published research earlier this year on a different kind of durable hydrophobic material, a rare-earth ceramic. Varanasi says that the two approaches will likely both find useful applications, but in different situations. The ceramic material can withstand even higher temperatures, while the new coating should be less expensive and appropriate for use in existing power plants.

The new coating can easily be applied to conventional condenser materials — typically titanium, steel, copper or aluminum — in existing facilities, using a process called initiated chemical vapor deposition (iCVD).

Another advantage of the new coating is that it can be extremely thin — just one-thousandth of the thickness of conventional hydrophobic coatings. That means other properties of the underlying surface, such as its electrical or thermal conductivity, are hardly affected.

Sumanta Acharya, the program director for the National Science Foundation's Thermal Transport Processes Program, who was not involved in this research, says, "In my opinion this work represents a major breakthrough in condenser technology. It offers the potential for significantly higher heat-transfer coefficients, high vapor-condensation rates and rapid removal of the condensate."

The U.S. Army Research Office, through MIT's Institute for Soldier Nanotechnologies, and the National Science Foundation supported the research.

This article is reprinted with permission of MIT News.