Researchers at MIT have developed a way of coating condenser surfaces in thermal power plants with a layer of graphene and found that this can improve the rate of heat transfer by a factor of four.
In laboratory tests the team also found the one atom thick coating to be highly durable compared to polymer coatings.
The findings are reported in Nano Letters by MIT graduate student Daniel Preston, professors Evelyn Wang and Jing Kong.
Condensers are used in power stations to turn the steam that drives turbines back into water so that the process can be repeated. An improvement in condenser heat transfer could lead to an overall improvement in power plant efficiency of 2 to 3 per cent based on figures from the Palo Alto, California-based Electric Power Research Institute, Preston said in a statement.
When the steam forms a film, Preston said, it impedes heat transfer – and reduces the efficiency – of condensation. The goal, therefore, 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 since we know it is hydrophobic by nature” said Preston.
So 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 vapour at 100 degrees Celsius.
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 optimising temperature differences could boost this improvement to 5 to 7 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 said, and failed completely within 12 hours.
Because the process used to coat the graphene on the copper surface – chemical vapour deposition – has been tested extensively, the new method could be ready for testing under real-world conditions in as little as a year, Preston said. And the process should be easily scalable to power plant-sized condenser coils.
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.