New solar material could clean drinking water

By U.S. Army CCDC Army Research Laboratory Public AffairsJuly 13, 2020

With Army funding researchers at the University of Rochester have developed an aluminum panel that angled at the sun purifies water.
With Army funding researchers at the University of Rochester have developed an aluminum panel that angled at the sun purifies water. (Photo Credit: Courtesy University of Rochester) VIEW ORIGINAL

RESEARCH TRIANGLE PARK, N.C. (July 13, 2020) – Providing clean water to Soldiers in the field and citizens around the world is essential, and yet one of the world’s greatest challenges. Now a new super-wicking and super-light-absorbing aluminum material developed with Army funding could change that.

With funding from the Army Research Office, an element of the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory, researchers at the University of Rochester have developed a new aluminum panel that more efficiently concentrates solar energy to evaporate and purify contaminated water.

“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 Dr. Evan Runnerstrom, program manager at ARO. “The combined super-wicking and light-absorbing properties of these aluminum surfaces may enable passive or low-power water purification to better sustain the warfighter in the field.”
The researchers developed a laser processing technology that turns regular aluminum pitch black, making it highly absorptive, as well as super-wicking. They then applied this super absorptive and super-wicking aluminum for solar water purification.
The researchers developed a laser processing technology that turns regular aluminum pitch black, making it highly absorptive, as well as super-wicking. They then applied this super absorptive and super-wicking aluminum for solar water purification. (Photo Credit: Courtesy University of Rochester) VIEW ORIGINAL

The researchers developed a laser processing technology that turns regular aluminum pitch black, making it highly absorptive, as well as super-wicking (it wicks water uphill against gravity). They then applied this super absorptive and super-wicking aluminum for this solar water purification.

The technology featured in Nature Sustainability, uses a burst of femtosecond (ultrashort) laser pulses to etch the surface of a normal sheet of aluminum. When the aluminum panel is dipped in water at an angle facing the sun, it draws a thin film of water upwards over the metal’s surface. At the same time, the blackened surface retains nearly 100-percent of the energy it absorbs from the sun to quickly heat the water. Finally, the wicking surface structures change the inter-molecular bonds of the water, 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,” said Professor Chunlei Guo, professor of optics at University of Rochester. “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 said.

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.

With Army funding researchers at the University of Rochester have developed an aluminum panel that angled at the sun purifies water.
With Army funding researchers at the University of Rochester have developed an aluminum panel that angled at the sun purifies water. (Photo Credit: Courtesy University of Rochester) VIEW ORIGINAL

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 said, 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. Furthermore, the available wicking materials become quickly clogged with contaminants left behind after evaporation, requiring frequent replacement of the materials.

The aluminum panel the researchers developed avoids these difficulties 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 said. “The biggest advantage 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.”

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.

In a paper in Nature Sustainability (read the paper: https://www.nature.com/articles/s41893-020-0566-x), researchers in the laboratory of Chunlei Guo, professor of optics (The Chunlei Guo Lab website: www.optics.rochester.edu/workgroups/guo), demonstrate how a burst of femtosecond laser pulses etch the surface of a normal sheet of aluminum into a superwicking (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.

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.

Experiments by the lab show that their new method reduces the presence of all common contaminants, such as detergent, dyes, urine, heavy metals, and glycerin, to safe levels for drinking.

Solar-based water purification can greatly reduce 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. 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.

Guo, who is also affiliated with the University’s Physics and Materials Science programs, has long envisioned an array of humanitarian applications for an efficient solar-based purification method. “This is a simple, durable, inexpensive way to address the global water crisis, especially in developing nations,” he says, noting that it could help relieve water shortages in drought-stricken areas and be helpful in water desalinization projects.

The project was supported by funding from the Bill and Melinda Gates Foundation, the National Science Foundation, and the US Army Research Office.

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The Army and Guo are exploring transition opportunities to further develop this technology within DOD laboratories and private industry.

In addition to the Army, this research received funding from the Bill and Melinda Gates Foundation and the National Science Foundation.

(Photo Credit: U.S. Army) VIEW ORIGINAL

CCDC Army Research Laboratory is an element of the U.S. Army Combat Capabilities Development Command. As the Army’s corporate research laboratory, ARL discovers, innovates and transitions science and technology to ensure dominant strategic land power. Through collaboration across the command’s core technical competencies, CCDC leads in the discovery, development and delivery of the technology-based capabilities required to make Soldiers more lethal to win the nation’s wars and come home safely. CCDC is a major subordinate command of the U.S. Army Futures Command.