Rice's "nanophotonics-enabled solar membrane distillation" (NESMD) technology was first demonstrated in 2012. It uses light-absorbing nanoparticles to turn a desalination membrane into a solar-driven heating element, enabling water purification without electricity. Recently, the team discovered it could boost the performance of the system by more than 50 per cent simply by adding inexpensive plastic lenses to concentrate sunlight into ‘hot spots’. The work is described in the Proceedings of the National Academy of Sciences.
"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."
The hot spots essentially deliver outsized gains compared to having the energy evenly distributed across the membrane. This process is akin to using a magnifying glass to concentrate the sun’s energy in a single spot, but results in a non-linear improvement in the membrane’s peformance. Focusing the light on a tiny spot on the membrane results in a linear increase in heat. However, the heating produces a nonlinear increase in vapour pressure, with the increased pressure forcing 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," said co-lead author Alessandro Alabastri, a physicist and Texas Instruments Research Assistant Professor in Rice's Department of Electrical and Computer Engineering.
According to the Rice team, this nonlinear optical effect also could improve technologies that use solar heating to drive chemical processes like photocatalysis