Lasers etch an efficient
way to address global water crisis
From University of Rochester
July 13, 2020 -- Researchers use
sunlight and a laser-etched metal surface to evaporate and purify water for
safe drinking at greater than 100 percent efficiency. The method could help
relieve water shortages in drought-stricken areas and be helpful in water
desalinization projects.
Amid the coronavirus pandemic, people in
developed countries are assured of ample supplies of clean water to wash their
hands as often as needed to protect themselves from the disease. And yet,
nearly a third of the world's population is not even assured of clean water for
drinking.
University of Rochester researchers have
now found a way to address this problem by using sunlight -- a resource that
everyone can access -- to evaporate and purify contaminated water with greater
than 100 percent efficiency.
How is this possible?
In a paper in Nature
Sustainability, researchers in the laboratory of Chunlei Guo, professor of
optics, demonstrate how a burst of femtosecond laser pulses etch the surface of
a normal sheet of aluminum into a super wicking (water attracting), super
energy absorbing material.
When placed in water at an angle facing
the sun, the surface:
- Draws
a thin film of water upwards over the metal's surface
- Retains
nearly 100 percent of the energy it absorbs from the sun to quickly heat
the water
- Simultaneously,
changes the inter-molecular bonds of the water, significantly increasing
the efficiency of the evaporation process even further.
"These three things together enable
the technology to operate better than an ideal device at 100 percent efficiency,"
says Guo, who is also affiliated with the University's Physics and Materials
Science programs. "This is a simple, durable, inexpensive way to address
the global water crisis, especially in developing nations."
Experiments by the lab show that the
method reduces the presence of all common contaminants, such as detergent,
dyes, urine, heavy metals, and glycerin, to safe levels for drinking.
The technology could also be useful in
developed countries for relieving water shortages in drought-stricken areas,
and for water desalinization projects, Guo says.
Easy to clean, easy to aim
Using sunlight to boil has long been
recognized as a way to eliminate microbial pathogens and reduce deaths from
diarrheal infections. But boiling water does not eliminate heavy metals and
other contaminants.
Solar-based water purification, however,
can greatly reduce these contaminants because nearly all the impurities are
left behind when the evaporating water becomes gaseous and then condenses and
gets collected.
The most common method of solar-based
water evaporation is volume heating, in which a large volume of water is heated
but only the top layer can evaporate. This is obviously inefficient, Guo says,
because only a small fraction of the heating energy gets used.
A more efficient approach, called
interfacial heating, places floating, multi-layered absorbing and wicking
materials on top of the water, so that only water near the surface needs to be
heated. But the available materials all have to float horizontally on top of the
water and cannot face the sun directly, Guo says. Thus, the approach is less
energy efficient. Furthermore, the available wicking materials become quickly
clogged with contaminants left behind after evaporation, requiring frequent
replacement of the materials.
The panel developed by the Guo lab
avoids these inefficiencies by pulling a thin layer of water out of the
reservoir and directly onto the solar absorber surface for heating and
evaporation. "Moreover, because we use an open-grooved surface, it is very
easy to clean by simply spraying it," Guo says.
"The biggest advantage," he
adds, "is that the angle of the panels can be continuously adjusted to
directly face the sun as it rises, and then moves across the sky before
setting" -- maximizing energy absorption. "There was simply nothing
else resembling what we can do here," Guo says.
Latest in series of applications
The project was supported by funding
from the Bill and Melinda Gates Foundation, the National Science Foundation,
and the US Army Research Office.
"The Army and its warfighters run
on water, so there is particular interest in basic materials research that
could lead to advanced technologies for generating drinking water," said
Evan Runnerstrom, program manager, Army Research Office, an element of the U.S.
Army Combat Capabilities Development Command's Army Research Laboratory.
"The superwicking and light-absorbing properties of these aluminum
surfaces may enable passive or low-power water purification to better sustain
the warfighter in the field."
In addition to using femto-second laser
etching technology to create superhydrophobic (water repellent),
superhydrophilic (water-attracting), and super energy absorbing metals, the Guo
lab has created metallic structures that do not sink no matter how often they
are forced into water or how much it is damaged or punctured.
Prior to creating the water attracting
and repellent metals, Guo and his assistant, Anatoliy Vorobyev, demonstrated
the use of femto-second laser pulses to turn almost any metal pitch black. The
surface structures created on the metal were incredibly effective at capturing
incoming radiation, such as light. But they also captured light over a broad
range of wavelengths.
Subsequently, his team used a similar
process to change the color of a range of metals to various colors, such as
blue, gold, and gray. The applications could include making color filters and
optical spectral devices, using a single laser in a car factory to produce cars
of different colors; or proposing with a gold engagement ring that matches the
color of your fiancee's blue eyes.
The lab also used the initial black and
colored metal technique to create a unique array of nano- and micro-scale
structures on the surface of a regular tungsten filament, enabling a light bulb
to glow more brightly at the same energy usage.
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