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Oct 24th, 2012
 
Patterned surfaces enhance condenser performance
 
Condensers are a crucial part of power generation systems: About 80 percent of all the world’s powerplants use them to turn steam back to water after it comes out of the turbines that turn generators.
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  • MIT researchers find that lubricated, nanotextured surfaces improved performance of condensers in power and desalination plants

They are also a key element in desalination plants, a fast-growing contributor to the world’s supply of fresh water.

Now, a new surface architecture designed by researchers at MIT holds the promise of significantly boosting the performance of such condensers.

The key to the improved hydrophobic (water-shedding) surface is a combination of microscopic patterning — a surface covered with tiny bumps or posts just 10 micrometers (millionths of a meter) across, about the size of a red blood cell — and a coating of a lubricant, such as oil. The tiny spaces between the posts hold the oil in place through capillary action, the researchers found.

The team discovered that droplets of water condensing on this surface moved 10,000 times faster than on surfaces with just the hydrophobic patterning. The speed of this droplet motion is key to allowing the droplets to fall from the surface so that new ones can form, increasing the efficiency of heat transfer in a powerplant condenser, or the rate of water production in a desalination plant.

The amount of lubricant required is minimal: It forms a thin coating, and is securely pinned in place by the posts. Any lubricant that is lost is easily replaced from a small reservoir at the edge of the surface.

Varanasi plans further research to quantify exactly how much improvement is possible by using the new technique in powerplants..

For the first time, this new technique obtains direct, detailed images of the interface between a surface and a liquid, such as droplets that condense on it. Normally, that interface — the key to understanding wetting and water-shedding processes — is hidden from view by the droplets themselves, Varanasi explains, so most analysis has relied on computer modeling. In the new process, droplets are rapidly frozen in place on the surface, sliced in cross-section with an ion beam, and then imaged using a scanning electron microscope.

The enhanced condensation research received funding from the National Science Foundation (NSF), the Masdar-MIT Energy Initiative program, and the MIT Deshpande Center. The direct imaging research used NIST facilities, with funding from an NSF grant and the Dupont-MIT Alliance.


 
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