Hot Spots Make Purifying Salt Water Super Efficient

A solar-powered way to purify salt water with sunlight and nanoparticles is even more efficient than first thought, a new study reports.

Simply adding inexpensive plastic lenses to concentrate sunlight into “hot spots” boosted the efficiency of a solar-powered desalination system by more than 50 percent, researchers say.

“The typical way to boost performance in solar-driven systems is to add solar concentrators and bring in more light,” says Pratiksha Dongare, a graduate student in applied physics at Rice University’s Brown School of Engineering and co-lead author of a new paper in PNAS.

“The big difference here is that we’re using the same amount of light. We’ve shown it’s possible to inexpensively redistribute that power and dramatically increase the rate of purified water production.”

Hot Spots Make Purifying Salt Water Super Efficient(Credit: Pratiksha Dongare/Rice)

Clean water

In conventional membrane distillation, hot, salty water is flowed across one side of a sheet-like membrane while cool, filtered water flows across the other. The temperature difference creates a difference in vapor pressure that drives water vapor from the heated side through the membrane toward the cooler, lower-pressure side.

Scaling up the technology is difficult because the temperature difference across the membrane—and the resulting output of clean water—decreases as the size of the membrane increases.

The new “nanophotonics-enabled solar membrane distillation” (NESMD) technology addresses this by using light-absorbing nanoparticles to turn the membrane itself into a solar-driven heating element.

Dongare and colleagues, including study co-lead author Alessandro Alabastri, coat the top layer of their membranes with low-cost, commercially available nanoparticles designed to convert more than 80 percent of sunlight energy into heat.

The solar-driven nanoparticle heating reduces production costs and engineers are working to scale up the technology for applications in remote areas that have no access to electricity.

Concentrated light

Director of Rice’s Laboratory for Nanophotonics Naomi Halas and research scientist Oara Neumann, both coauthors of the new study, first demonstrated the concept and particles used in the technology in 2012. In the new study, the scientists found they could exploit an inherent and previously unrecognized nonlinear relationship between incident light intensity and vapor pressure.

Alabastri, a physicist and research assistant professor in the electrical and computer engineering department, used a simple mathematical example to describe the difference between a linear and nonlinear relationship.

“If you take any two numbers that equal 10—7 and 3, 5 and 5, 6 and 4—you will always get 10 if you add them together. But if the process is nonlinear, you might square them or even cube them before adding. So if we have 9 and 1, that would be 9 squared, or 81, plus one squared, which equals 82. That is far better than 10, which is the best you can do with a linear relationship.”

In the case of NESMD, the nonlinear improvement comes from concentrating sunlight into tiny spots, much like a child might with a magnifying glass on a sunny day. Concentrating the light on a tiny spot on the membrane results in a linear increase in heat, but the heating, in turn, produces a nonlinear increase in vapor pressure. And the increased pressure forces more purified steam through the membrane in less time.

“We showed that it’s always better to have more photons in a smaller area than to have a homogeneous distribution of photons across the entire membrane,” Alabastri says.

“The efficiencies provided by this nonlinear optical process are important because water scarcity is a daily reality for about half of the world’s people, and efficient solar distillation could change that.” Halas says.

“Beyond water purification, this nonlinear optical effect also could improve technologies that use solar heating to drive chemical processes like photocatalysis.”

For example, LANP is developing a copper-based nanoparticle for converting ammonia into hydrogen fuel at ambient pressure.

Halas is a professor of electrical and computer engineering, a professor of chemistry, bioengineering, physics and astronomy, and materials science and nanoengineering and director of Rice’s Smalley-Curl Institute.

The National Science Foundation, the Air Force Office of Scientific Research, the Welch Foundation, and the Smalley-Curl Institute funded the work. Dongare, Alabastri, Neumann, Nordlander, and Halas are co-inventors on a provisional patent relating to the research.

Source: Rice University

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