Most of the world’s electricity-producing power plants — whether powered by coal, natural gas or nuclear fission — make electricity by generating steam that turns a turbine. That steam then is condensed back to water, and the cycle begins again. But, the condensers that collect the steam are quite inefficient, and improving them could make a big difference in overall power plant efficiency.
A team of researchers at MIT has developed a way of coating these condenser surfaces with a layer of graphene — just one atom thick — and found that this can improve the rate of heat transfer by a factor of four. (Even better heat transfer gains may be possible with further work.) And unlike polymer coatings, the graphene coatings have proven to be highly durable in laboratory tests. The findings are reported in the journalNano Letters by MIT graduate student Daniel Preston, professors Evelyn Wang and Jing Kong, and two others.
The improvement in condenser heat transfer, which is just one step in the power-production cycle, could lead to an overall improvement in power plant efficiency of 2 to 3 percent, based on figures from the Electric Power Research Institute, Preston says. This is enough to make a significant dent in global carbon emissions, he notes, because such plants represent the majority of the world’s electricity generation. “That translates into millions of dollars per power plant per year,” he explains.
There are two basic ways in which the condensers 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 explains, that impedes heat transfer — and thus reduces the efficiency of condensation. So, 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. 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.” He and his colleagues decided to test both graphene’s ability to shed water and its durability under typical power plant conditions — an environment of pure water vapor at 212°F (100°C).
They 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 five to seven times. The researchers also showed that after two full 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 just three hours, Preston says, and failed completely within 12 hours.
Because the chemical vapor deposition process used to coat the graphene on the copper surface has been tested extensively, the new method could be ready for testing under real-world conditions “in as little as a year,” Preston says. The researchers expect the process to be scalable to power plant-sized condenser coils.
“This work is extremely significant because, to my knowledge, it is the first report of durable dropwise condensation with a single-layer surface coating,” says Jonathan Boreyko, an assistant professor of biomedical engineering and mechanics at Virginia Tech who has studied condensation on superhydrophobic surface. “These findings are somewhat surprising and very exciting.”
Boreyko, who was not involved in the research, adds that this method, if proven through further testing, “could significantly improve the efficiency of power plants and other systems that utilize condensers.”
The research team also included MIT postdoc Daniela Mafra and former postdoc Nenad Miljkovic, who is now an assistant professor at the University of Illinois at Urbana-Champaign. The Office of Naval Research and the National Science Foundation supported the work.