The solar-powered approach of Rice University to the purification of salt water with sunlight and nanoparticles is even more effective than its creators thought first.
Researchers at Rice's Laboratory for Nanophotonics (LANP) this week showed that they could increase the efficiency of their solar powered desalination system by more than 50% by simply adding cheap plastic lenses to concentrate sunlight to "hot spots ". The results are available online in Proceedings of the National Academy of Sciences .
"The typical way to increase performance in solar powered 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 and co-author of the paper. "The big difference here is that we use the same amount of light. We have shown that it is possible to redistribute this stream cheaply and dramatically increase the rate of cleaning water." In conventional membrane distillation, hot, salt water is flowed over one side of a sheet-like membrane, while cool filtered water flows over the other. The temperature difference creates a difference in vapor pressure which drives water vapor from the heated side through the membrane toward the lower side of the cooler. Upscaling of the technology is difficult because the temperature difference across the membrane ̵
Dongars and colleagues, including study director Alessandro Alabastri, coat their top layers of membranes with cheap and commercially available nanoparticles designed to convert more than 80% sunlight energy to heat. The solar powered nanoparticle heat reduces production costs, and Rice engineers are working to scale up the technology for applications in remote areas that do not have access to electricity.
The concept and particles used in NESMD were first demonstrated in 2012 by LANP director Naomi Halas and researcher Oara Neumann, both co-authors of the new study. In this week's study, Halas, Dongare, Alabastri, Neumann and LANP physicist Peter Nordlander found that they could utilize an inherent and previously unrecognized non-linear relationship between incident light intensity and vapor pressure.
Alabastri, a physicist and Texas Instruments Research Assistant Professor of Rice's Department of Electrical and Computer Engineering, used a simple mathematical example to describe the difference between a linear and non-linear relationship. "If you take two numbers corresponding to 10 and three, five and five, six and four, you will always get 10 if you add them together. But if the process is not linear, you can square them or even cube them before Then If we have nine and one, it would be nine squared or 81 plus one squared, which is 82. It's far better than 10, which is the best you can do with a linear relationship. "
In case of NESMD comes the nonlinear improvement from concentrating sunlight to small spots, just as a child can with a magnifying glass on a sunny day. Concentration of the light on a small spot on the membrane results in a linear increase in heat, but the heating again produces a non-linear increase in vapor pressure. And the increased pressure forces more purified steam through the membrane in less time.
"We showed that it is always better to have more photons in a smaller area than to have a homogeneous distribution of photons across the membrane," Alabastri said.
Halas, a chemist and engineer who has used more than 25 years of groundbreaking use of light-activated nanomaterials, said: "The effectiveness of this non-linear optical process is important because water scarcity is a daily reality for about half of the world "Beyond water purification, this non-linear optical effect could also improve technologies that use solar heat to drive chemical processes such as photocatalysis," Halas says.
LANP, for example, develops a copper-based nanoparticle for the conversion of ammonia to hydrogen fuel at ambient pressure
Halas is Stanley C. Moore Professor of Electrical Engineering and Computer Engineering, Director of Rice's Smalley-Curl Institute and Professor of Chemistry, Bioengineering, Physics and Astronomy and material science and nanoengineering.
NESMD is developing at the rice-based center for nanotechnologically activated water treatment (N EWT) and won research and development funding from the Department of Energy's Solar Desalination Program in 2018.
Fresh water from salt water using only solar energy: Modular technology for desalination of off-grid
Pratiksha D. Dongare et al., Solar thermal desalination as a nonlinear optical process, Procedures of the National Academy of Sciences (2019). DOI: 10.1073 / pnas.1905311116
Engineers increase production of solar desalination system by 50% (2019, June 18)
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