Condensers are a crucial part of today’s 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.
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 research is described in a paper just published online in the
journal ACS Nano by MIT postdoc Sushant Anand; Kripa Varanasi, the
Doherty Associate Professor of Ocean Utilization; and graduate student
Adam Paxson, postdoc Rajeev Dhiman and research affiliate Dave Smith,
all of Varanasi’s research group at MIT.
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.
With this new treatment, “drops can glide on the surface,” Varanasi
says, floating like pucks on an air-hockey table and looking like
hovering UFOs — a behavior Varanasi says he has never seen in more than
a decade of work on hydrophobic surfaces. “These are just crazy
velocities.”
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. The lubricant can be designed to have such low vapor
pressure that, Varanasi says, “You can even put it in a vacuum, and it
won’t evaporate.”
Another advantage of the new system is that it doesn’t depend on any
particular configuration of the tiny textures on the surface, as long
as they have about the right dimensions. “It can be manufactured
easily,” Varanasi says. After the surface is textured, the material can
be mechanically dipped in the lubricant and pulled out; most of the
lubricant simply drains off, and “only the liquid in the cavities is
held in by capillary forces,” Anand says. Because the coating is so
thin, he says, it only takes about a quarter- to a half-teaspoon of
lubricant to coat a square yard of the material. The lubricant can also
protect the underlying metal surface from corrosion.
Varanasi plans further research to quantify exactly how much
improvement is possible by using the new technique in powerplants.
Because steam-powered turbines are ubiquitous in the world’s
fossil-fuel powerplants, he says, “even if it saves 1 percent, that’s
huge” in its potential impact on global emissions of greenhouse gases.
The new approach works with a wide variety of surface textures and
lubricants, the researchers say; they plan to focus ongoing research on
finding optimal combinations for cost and durability. “There’s a lot of
science in how you design these liquids and textures,” Varanasi says.
That further research will be aided by a new technique Varanasi has
developed in collaboration with researchers including Konrad
Rykaczewski, an MIT research scientist currently based at the National
Institute of Standards and Technology (NIST) in Gaithersberg, Md.,
along with John Henry Scott and Marlon Walker of NIST and Trevan Landin
of FEI Company. That technique is described in a separate paper also
just published in ACS Nano.
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 method relies on preserving the geometry of the samples through
rapid freezing in liquid-nitrogen slush at minus 210 degrees Celsius
[minus 346 degrees Fahrenheit],” Rykaczewski says. “The freezing rate
is so fast (about 20,000 degrees Celsius per second) that water and
other liquids do not crystalize, and their geometry is preserved.”
The technique could be used to study many different interactions
between liquids or gases and solid surfaces, Varanasi says. “It’s a
completely new technique. For the first time, we’re able to see these
details of these surfaces.”
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.
Source: MIT