Nonlinear Optical Process Could Improve Solar-Powered Desalination

Researchers at Rice University have improved the efficiency of their solar-powered desalination system by more than 50% by concentrating sunlight into “hot spots.” Using the same amount of light, the researchers were able to increase the rate of purified water production by redistributing direct sunlight intensity using inexpensive plastic lenses rather than by increasing overall intensity with large solar concentrators.

To increase the efficiency of their system, the researchers developed a nanophotonics-enabled solar membrane distillation (NESMD) technology that uses light-absorbing nanoparticles to turn the distillation membrane into a solar-driven heating element. The top layer of the membranes are coated with low-cost, commercially available nanoparticles that are designed to convert more than 80% of sunlight energy into heat. The particles that are used, and the concept that NESMD is based on, exploit the nonlinear relationship between incident light intensity and vapor pressure.

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. Courtesy of Pratiksha Dongare/Rice University.

With NESMD, the nonlinear improvement comes from concentrating sunlight into tiny spots, much like a using a magnifying glass on a sunny day. Concentrating the light on a tiny spot of the membrane results in a linear increase in the amount of heat, which produces a nonlinear increase in vapor pressure. The increase in vapor pressure forces more purified steam through the membrane in less time, because the purified water output is exponentially dependent on the light intensity.

Applying optical nonlinearity to desalination design, rather than increasing solar collector size, could lead to higher-performance solar water-purification systems within a small footprint, suitable for portability and use in remote locations.

Rice University researchers (from left) Pratiksha Dongare, Alessandro Alabastri, and Oara Neumann showed that Rice’s nanophotonics-enabled solar membrane distillation (NESMD) system was more efficient when the light was concentrated in hot spots. Courtesy of Jeff Fitlow/Rice University.

“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,” researcher Alessandro Alabastri said.

“Beyond water purification, this nonlinear optical effect also could improve technologies that use solar heating to drive chemical processes like photocatalysis,” said professor Naomi Halas, director of Rice’s Laboratory for Nanophotonics.

The research was published in the Proceedings of the National Academy of Sciences (https://doi.org/10.1073/pnas.1905311116).

The desalination system, which uses a combination of membrane distillation technology and light-harvesting nanophotonics, is the first major innovation from the Center for Nanotechnology Enabled Water Treatment (NEWT), a multi-institutional engineering research center based at Rice University. Courtesy of Rice University.