By Dr. TieJun Zhang and Abulimiti Aili
Steam-driven power generation may seem straightforward – a liquid is heated, producing steam that turns a turbine, and power is generated. But what happens after that point – when that steam is condensed back into liquid by rejecting heat to the ambient – is difficult, particularly in hot and arid regions.
Condensers, which collect and condense the steam, help close the power cycle and in turn play an important role in a power plant’s overall efficiency. According to an MIT news story released last year, condenser improvements could increase power plant efficiency by 3%, which is considered enough to significantly reduce global carbon emissions given that most of the world’s power generation is thermally driven. Unfortunately, most condensers used today are not very efficient, which is why our team is exploring ways to improve condensation processes through the development of innovative technologies.
Filmwise condensation is the most common and least efficient type of condensation used in traditional condensers. In filmwise condensation, liquid film condensate builds up on the condenser, creating a barrier between the vapor and the cold condenser surface.
A more efficient condensation process is dropwise condensation on hydrophobic (water-repellent) surfaces, where the vapor is condensed into individual droplets which can be easily shed away by gravity.
An even more efficient dropwise condensation process is condensation on superhydrophobic (super-water-repellent) surfaces, where microscopic-sized water droplets coalesce together and jump away from the condenser surface without the need of gravity. This superhydrophobic surface is achieved by introducing nanostructures to a condenser’s surface and then coating it with hydrophobic materials.
In our work, which was recently published in the journal ACS Applied Materials & Interfaces, we have proposed an even more exciting approach to promote droplet jumping by adding micro-pores, or microscopic holes, onto the superhydrophobic surface of a condenser. The micro-pores, which are made by introducing a low-cost copper micro-mesh to the condenser’s surface, squeeze the condensing droplets, speeding up the droplets’ unidirectional growth, which then triggers the droplets to jump away from the surface. This fast droplet growth and jumping away drastically reduces the condenser’s thermal resistance and further improves the heat transfer rate.
The nanostructured micro-mesh helps speed up the growth and jumping away of the water droplets, causing the drops to fall away before they become too large. This is important because the bigger the droplets become means there is a barrier for new droplets to form. The nanostructured micro-mesh we developed causes the water droplets to jump away from the condenser surface while they are still small (ranging in size from 10 to 100 microns). This helps keep the rate of heat transfer high and significantly improves the condenser’s performance.
Another important advantage of the copper micro-mesh is that it is affordable and easy to fabricate, making this technology easily scalable for industrial applications. In addition to improving condenser efficiencies in steam-driven power plants, this novel condensation mechanism could also make other technologies that rely on condensation, including desalination, more affordable and efficient.
We are excited by this new phenomenon and we plan to further investigate its underlying physics in order to understand it better and discover more ways in which this technology can contribute to greater sustainability in the UAE and around the world.
Dr. TieJun Zhang is Assistant Professor of Mechanical and Materials Engineering at Masdar Institute; and Abulimiti Aili is a Masdar Institute Class of 2016 MSc in Mechanical and Materials Engineering graduate and current Research Engineer.