Researchers in Rice University's Laboratory for Nanophotonics or LANP this week showed that they could boost the efficiency of their solar-powered desalination system by more than 50% 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 and co-lead author of the paper. "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, salty, hot water has flowed across one side of a membrane that is sheet-like while filtered, cool 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 and lower-pressured side.
Scaling up the technology is difficult; it is because the temperature difference across the membrane decreases as the size of the membrane increases. Rice University's nanophotonics-enabled solar membrane distillation or NESMD technology addresses this by using nanoparticles that are light-absorbing to turn the membrane itself into a solar-driven heating element.
Pratiksha Dongare and her colleagues, Allesandro Alabastri, the co-lead author of the study, 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 nanoparticle heating that is solar driven reduces production costs, and Rice engineers are working to scale up the technology for applications in remote areas that have no access to electricity.
This concept used in NESMD was first shown in 2012 by LANP director Naomi Halas and research scientist Oara Neumann, who are both co-authors on the new study. In this week's study, Halas, Alabastri, Dongare, 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, 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."
