Discovery of Charged Droplets Could Lead to More Efficient Power Plants
Oct. 2, 2013 — In a
completely unexpected finding, MIT researchers have discovered that tiny
water droplets that form on a superhydrophobic surface, and then "jump"
away from that surface, carry an electric charge. The finding could
lead to more efficient power plants and a new way of drawing power from
the atmosphere, they say.
Images such
as this, showing droplets being shed from a superhydrophobic surface
(light band at center), revealed the charging of the droplets. (Credit:
Nenad Miljkovic and Daniel Preston)
The finding is reported in a paper in the journal Nature Communications written by MIT postdoc Nenad Miljkovic, mechanical engineering professor Evelyn Wang, and two others.
Miljkovic says this was an extension of previous work by the MIT
team. That work showed that under certain conditions, rather than simply
sliding down and separating from a surface due to gravity, droplets can
actually leap away from it. This occurs when droplets of water condense
onto a metal surface with a specific kind of superhydrophobic coating
and at least two of the droplets coalesce: They can then spontaneously
jump from the surface, as a result of a release of excess surface
energy.
In the new work, "We found that when these droplets jump, through
analysis of high-speed video, we saw that they repel one another
midflight," Miljkovic says. "Previous studies have shown no such effect.
When we first saw that, we were intrigued."
In order to understand the reason for the repulsion between jumping
droplets after they leave the surface, the researchers performed a
series of experiments using a charged electrode. Sure enough, when the
electrode had a positive charge, droplets were repelled by it as well as
by each other; when it had a negative charge, the droplets were drawn
toward it. This established that the effect was caused by a net positive
electrical charge forming on the droplets as they jumped away from the
surface.
The charging process takes place because as droplets form on a
surface, Miljkovic says, they naturally form an electric double layer --
a layer of paired positive and negative charges -- on their surfaces.
When neighboring drops coalesce, which leads to their jumping from the
surface, that process happens "so fast that the charge separates," he
says. "It leaves a bit of charge on the droplet, and the rest on the
surface."
The initial finding that droplets could jump from a condenser surface
-- a component at the heart of most of the world's
electricity-generating power plants -- provided a mechanism for
enhancing the efficiency of heat transfer on those condensers, and thus
improving power plants' overall efficiency. The new finding now provides
a way of enhancing that efficiency even more: By applying the
appropriate charge to a nearby metal plate, jumping droplets can be
pulled away from the surface, reducing the likelihood of their being
pushed back onto the condenser either by gravity or by the drag created
by the flow of the surrounding vapor toward the surface, Miljkovic says.
"Now we can use an external electric field to mitigate" any tendency
of the droplets to return to the condenser, "and enhance the heat
transfer," he says.
But the finding also suggests another possible new application,
Miljkovic says: By placing two parallel metal plates out in the open,
with "one surface that has droplets jumping, and another that collects
them … you could generate some power" just from condensation from the
ambient air. All that would be needed is a way of keeping the condenser
surface cool, such as water from a nearby lake or river. "You just need a
cold surface in a moist environment," he says. "We're working on
demonstrating this concept."
The research team also included graduate student Daniel Preston and
Ryan Enright, who was a postdoc at MIT and the University of Limerick
and is now at Bell Labs Ireland, part of Alcatel-Lucent. The work
received funding from the U.S. Department of Energy through the MIT
Solid-State Solar-Thermal Energy Conversion Center, the Office of Naval
Research and the National Science Foundation.
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