According to the latest issue of the Proceedings of the National Academy of Sciences, Rice University in the United States uses cheap plastic lenses to focus sunlight to "hot spots", increasing the efficiency of solar water desalination systems by more than 50%.
Researchers at the Rice University Nanophotonics Laboratory (LNAP) said that the typical way to improve the performance of solar desalination systems is to add solar concentrators and increase light. The biggest difference of the new method is that using the same amount of light can also redistribute power at low cost and greatly increase the productivity of pure water.
In traditional membrane distillation, hot brine flows through one side of the sheet membrane, while cooling filtered water flows through the other side. The temperature difference produces a vapor pressure difference that drives water vapor from the heated side through the membrane to the cooler low-pressure side. The disadvantage of this technique is that the temperature difference of the membrane and the resulting clean water output decrease as the membrane size increases. Rice University's newly developed Nano Photonic Solar Film Distillation (NESMD) technology uses light-absorbing nanoparticles to transform the film itself into a solar-powered heating element, which solves this problem.
NESMD technology utilizes the inherently unknown nonlinear relationship between incident light intensity and vapor pressure. The nonlinear improvement comes from focusing the sunlight into tiny spots. Concentrating light through the lens on the tiny spots on the membrane causes a linear increase in heat, but heating in turn produces a non-linear increase in vapor pressure. The increased pressure forces more purified vapor to pass through the membrane in a shorter time. Research has found that it is always better to have more photons in a smaller area than to distribute photons evenly throughout the film.
Researchers say that because more than half of the world ’s population is in a state of water shortage, nonlinear high-efficiency solar distillation technology can greatly improve the lives of these people. In addition to water purification, this nonlinear optical effect can also use solar heating to drive chemical processes such as photocatalysis. LNAP is developing a copper-based nanoparticle for converting ammonia into hydrogen fuel under ambient pressure. (Reporter Feng Weidong)
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