Rice University's solar-powered approach to purifying salt water with sunlight and nanoparticles is even more efficient than the inventors initially believed.
Researchers at Rice's Nanophotonics Laboratory (LANP) this week have shown they can increase the efficiency of their solar-powered desalination system by more than 50% by simply adding low-cost plastic lenses to focus sunlight on "hot spots" , The results are available online in the Proceedings of the National Academy of Sciences .
"The typical way to increase performance in solar powered systems is to add solar concentrators and introduce more light," said Pratiksha Dongare, a PhD student in applied physics at the Rice Brown School of Engineering and co-principal author of the thesis. "The big difference is that we use the same amount of light, and we've shown that it's possible to redistribute this power cost-effectively and drastically increase the rate of purified water production."
In conventional membrane distillation, hot, salty water flows over one side of a sheet-like membrane, while cold, filtered water flows over the other side. The temperature differential creates a vapor pressure differential that drives water vapor from the heated side through the membrane to the cooler side at lower pressure. The technology is difficult to scale because the temperature difference between the membranes and the resulting clean water output decreases with increasing membrane size. Rice's "Nanophotonics-enabled Solar Membrane Distillation" (NESMD) technology addresses this problem by transforming the membrane itself into a solar-powered heating element using light-absorbing nanoparticles. Rice The university researchers (from left) Pratiksha Dongare, Alessandro Alabastri and Oara Neumann showed that the NESMD (Nanophotonics-enabled Solar Membrane Distillation) system from Rice was more efficient when the size of the device was increased and the light on "Hot Spots "was concentrated. & # 39; Picture credits: Jeff Fitlow / Rice University
Dongare and colleagues, including the co-leader of the study, Alessandro Alabastri, coat the top layer of their membranes with low-cost, commercially available nanoparticles designed to convert more than 80% of solar energy to heat. Solar-powered nanoparticle warming lowers production costs, and Rice engineers are working to expand the technology for remote area applications where there is no access to electricity. Naomi Halas and research scientist 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 exploit an inherent and previously unrecognized nonlinear relationship between incident light intensity and vapor pressure.
Alabastri, physicist and Texas Instruments Research Assistant Professor in Rice's Department of Electrical Engineering and Information Technology used a simple mathematical example to describe the difference between a linear and a nonlinear relationship. "If you take two numbers that are equal to 1
0 – seven and three, five and five, six and four – you always get 10 if you add them, but if the process is nonlinear, you can square them or even roll before So if we have nine and one, that would be nine square, or 81 plus one square, which equals 82. That's far better than 10, which is the best you can do with a linear relationship. "
In the case of NESMD is the non-linear improvement in that the sunlight is focused on tiny dots, much like a child on a sunny day with a magnifying glass. The concentration of light at a minute point on the membrane results in a linear increase in heat, but heating in turn produces a non-linear increase in vapor pressure. And the increased pressure forces more purified vapor through the membrane in less time.
"We have shown that it is always better to have more photons in a smaller area than a homogeneous distribution of photons across the entire membrane," said Alabastri.
Halas, a chemist and engineer who has worked for more than 25 years "The efficiency of this nonlinear optical process is important because water scarcity is a daily reality for about half the world's population and efficient solar distillation could change that." This nonlinear optical effect can also be used to improve technologies that use solar heat to perform chemical processes such as photocatalysis, "says Halas.
LANP, for example, develops a copper-based nanoparticle to convert ammonia to hydrogen at ambient pressure.
Halas is Stanley C. Moore Professor of Electrical Engineering and Information Technology, Director of the Smalley Curl Institute of Rice and Professor of Chemistry, Bioeng Ineering, Physics and Astronomy, and Materials Science and Nanotechnology.
NESMD is located at the Rice-based Center for Nanotechnology Enabled Water Treatment (NEWT) in development and was funded by the Department of Energy in 2018 with research and development funding for the solar desalination program.
Fresh water from salt water that uses only solar energy: Modular off-grid desalination technology
Pratiksha D. Dongare et al., Solarthermal Desalination as a Nonlinear Optical Process, Proceedings of the National Academy of Sciences (2019). DOI: 10.1073 / pnas.1905311116
Engineers increase solar desalination power by 50% (2019, June 18)
retrieved on 18 June 2019
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