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Researchers at the Massachusetts Institute of Technology have developed a hydrophobic condensing surface with hundreds of nanoscale projections that are said to help increase heat transfer by 30% compared to other hydrophobic surfaces available.
The researchers said that thousands of commercial power plants worldwide could be retrofitted with the technology to make them more efficient.
In power plants, water vapour condenses on to a metal plate, and then runs off and is returned to the boiler to be vaporised again. To make this process more efficient, the water vapour needs to fall on to and run off the metal plate as easily as possible.
Boosting the efficiency of this process can be achieved by increasing the plate’s heat-transfer coefficient – a measure of how readily heat is transferred from a surface. Nenad Miljkovic, a doctoral student at the university, and colleagues in the device research laboratory, have designed a surface coating that has an increased heat-transfer coefficient.
The nanostructured pattern causes the water droplets to jump away from the surface, rather than running off. This is possible as the tiny droplets merge to form larger ones and release energy that propels the water upwards from the surface.
Typically, water droplets form a liquid film on a flat-plate condenser that prevents any other vapour condensing until the water has run off, and this acts as a barrier to transferring heat. By removing the droplets of water as quickly as possible, the new surface can transfer heat more efficiently.
Previously, scientists have developed nanopatterned surfaces that induce this jumping effect, but the process of manufacturing them has proved complex, and usually requires a clean room. And these earlier attempts could only be used on flat surfaces, but in condensers tubing and other shapes are used.
Miljkovic said: “Now, we’ve gone a step further, developing a surface that favours these kinds of droplets, while being highly scaleable. Also, we’ve been able to experimentally measure the heat-transfer enhancement.”
The pattern is created using a wet-oxidisation process on the surface, which can be applied to the copper tubes and plates that are used in power plants. It comprises copper oxide and forms on top of the copper tubing. The reaction is self-limiting at room temperature.
The treatment creates a surface that resembles a bed of tiny, pointed leaves sticking upwards. It is these projections that stop the liquid from forming a film on the condenser.
Once the pointy pattern is created as a hydrophobic coating, the scientists apply a vapour solution that bonds to the surface.
Tests in the laboratory have shown that the heat-transfer efficiency of the new surface is 30% higher than the best hydrophobic condensing surfaces available today, said the researchers. They added that the surface could also be used in other heat-transfer applications, including dehumidifiers and the heating and cooling systems of buildings.
There are challenges with using the new approach. For example, if too many droplets form on the surface it can cause a flood that reduces the heat-transfer ability of the material. But the researchers are trying to delay this flooding to create a coating that can work well under all conditions.