Power Plant Efficiency Could Get a Boost from Condenser Coating
Engineering360 News Desk | June 02, 2015Researchers at the Massachusetts Institute of Technology (MIT) have developed a way to improve heat transfer from steam condensers in electricity producing power plants by a factor of four by coating condenser surfaces with a one-atom-thick layer of graphene.
Unlike polymer coatings, the graphene coatings have proven to be highly durable in laboratory tests. The findings are reported in the journal Nano Letters by MIT graduate student Daniel Preston, professors Evelyn Wang and Jing Kong and others. The improvement in condenser heat transfer could lead to an overall improvement in power plant efficiency of 2-3%, Preston says.
An uncoated copper condenser tube (top left) is shown next to a similar tube coated with graphene (top right). When exposed to water vapor at 100 degrees Celsius, the uncoated tube produces an inefficient water film (bottom left), while the coated shows the more desirable dropwise condensation (bottom right). An uncoated copper condenser tube (top left) is shown next to a similar tube coated with graphene (top right). When exposed to water vapor at 100 degrees Celsius, the uncoated tube produces an inefficient water film (bottom left), while the coated shows the more desirable dropwise condensation (bottom right).Image credit: MIT researchersThere are two basic ways in which the condensers—which may take the form of coiled metal tubes, often made of copper—interact with the flow of steam. In some cases, the steam condenses to form a thin sheet of water that coats the surface; in others it forms water droplets that are pulled from the surface by gravity.
When the steam forms a film, Preston says, that impedes heat transfer—and thus reduces the efficiency—of condensation. The goal of much research has been to enhance droplet formation on these surfaces by making them water-repelling.
Often this has been accomplished using polymer coatings, but these tend to degrade rapidly in the high heat and humidity of a power plant. And when the coatings are made thicker to reduce that degradation, the coatings themselves impede heat transfer.
"We thought graphene could be useful," Preston says, "since we know it is hydrophobic by nature."
Preston and his colleagues found that the single-atom-thick coating of graphene improved heat transfer fourfold compared with surfaces where the condensate forms sheets of water, such as bare metals. Further calculations showed that optimizing temperature differences could boost this improvement to 5 to 7 times. The researchers also showed that after two weeks under such conditions, there was no measurable degradation in the graphene's performance.
By comparison, similar tests using a common water-repelling coating showed that the coating began to degrade within three hours, Preston says, and failed completely within 12 hours.
Because the process used to coat the graphene on the copper surface—called chemical vapor deposition—has been tested, the new method could be ready for testing under real-world conditions "in as little as a year," Preston says. He says the process should be “easily scalable” to power plant-sized condenser coils.