Hot Spots Increase Efficiency of Solar Desalination

Concentrating the sunlight on tiny spots on the heat-generating membrane exploits an inherent and previously unrecognized nonlinear relationship between photothermal heating and vapor pressure. (Source: P. Dongare, Rice U.)

Researchers in Rice’s Laboratory for Nano­photonics (LANP) showed they could boost the effi­ciency of their solar-powered desali­nation system by more than 50% simply by adding inexpensive plastic lenses to concen­trate sunlight into hot spots. “The typical way to boost performance in solar-driven systems is to add solar concentrators and bring in more light,” said Pratiksha Dongare, a graduate student in applied physics at Rice’s Brown School of Engineering. “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.”

In conventional membrane distil­lation, hot, salty water is flowed across one side of a sheetlike 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. Rice’s 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 Alessandro Alabastri, coat the top layer of their membranes with low-cost, commercially available nano­particles that are designed to convert more than 80% of sunlight energy into heat. The solar-driven nano­particle heating reduces production costs, and Rice engineers are working to scale up the technology for appli­cations in remote areas that have no access to elec­tricity. The concept and particles used in NESMD were first demonstrated in 2012 by LANP director Naomi Halas and research scientist Oara Neumann. Now, Halas, Dongare, Alabastri, Neumann and LANP physicist Peter Nordlander found they could exploit an inherent and previously unrecog­nized nonlinear relation­ship between incident light intensity and vapor pressure.

Alabastri, a physicist and Texas Instruments Research Assistant Professor in Rice’s Department of Electrical and Computer Engineering, used a simple mathe­matical example to describe the difference between a linear and nonlinear relation­ship. “If you take any two numbers that equal 10 – seven and three, five and five, six and four – 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 nine and one, that would be nine 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 said.

Halas, a chemist and engineer who’s spent more than 25 years pioneering the use of light-activated nano­materials, said, “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. 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. (Source: Rice U.)

Reference: P. D. Dongare et al.: Solar thermal desalination as a nonlinear optical process, Proc. nat. Ac. Sc., online 17 June 2019; DOI: 10.1073/pnas.1905311116

Link: Nanoengineered Photonics and Plasmonics (Naomi Halas), Rice University, Houston, USA

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