Increasing the output of solar desalination by 50%

Dongare and colleagues, including study co-lead author 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, who are both co-authors on the new study. In this week’s study, 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,” Halas said.

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

Learn more: ‘Hot spots’ increase efficiency of solar desalination