Highly efficient and salt rejecting solar evaporation via a wick-free confined water layer

Lenan Zhang,Arny Leroy,Lin Zhao,Evelyn N Wang,Zhenyuan Xu,Yang Zhong,Xiangyu Li

doi:10.1038/s41467-022-28457-8

Lenan Zhang, Arny Leroy + Show 5 more

Open Access

https://doi.org/10.1038/s41467-022-28457-8

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Abstract

Recent advances in thermally localized solar evaporation hold significant promise for vapor generation, seawater desalination, wastewater treatment, and medical sterilization. However, salt accumulation is one of the key bottlenecks for reliable adoption. Here, we demonstrate highly efficient (>80% solar-to-vapor conversion efficiency) and salt rejecting (20 weight % salinity) solar evaporation by engineering the fluidic flow in a wick-free confined water layer. With mechanistic modeling and experimental characterization of salt transport, we show that natural convection can be triggered in the confined water. More notably, there exists a regime enabling simultaneous thermal localization and salt rejection, i.e., natural convection significantly accelerates salt rejection while inducing negligible additional heat loss. Furthermore, we show the broad applicability by integrating this confined water layer with a recently developed contactless solar evaporator and report an improved efficiency. This work elucidates the fundamentals of salt transport and offers a low-cost strategy for high-performance solar evaporation.

Highlights

Recent advances in thermally localized solar evaporation hold significant promise for vapor generation, seawater desalination, wastewater treatment, and medical sterilization
Since solarthermal conversion is localized within the water layer and heat loss to the bulk water is blocked by the thermal insulation, high thermal localization, comparable to the wick structure-based solar evaporation, can be achieved
We tested the normal mode evaporator because it is widely used for desalination, while requiring improved salt rejecting performance. We applied it to the contactless mode evaporator, where the solar absorber is above the confined water layer and IR absorption occurs at the water–air interface (Fig. 1f)

Summary

Introduction

Recent advances in thermally localized solar evaporation hold significant promise for vapor generation, seawater desalination, wastewater treatment, and medical sterilization. Due to the ultralow diffusivity of salt in water (~10−9 m2 s−1, as a reference in comparison to the diffusivity of vapor in air is ~10−5 m2 s−1), there is significant salt accumulation, which induces undesirable fouling, reduces evaporation rate, and degrades device reliability This effect has become one of the key practical challenges for a range of applications[8,20–26]. Instead of pursuing extreme interfacial water confinement using wick structures or eliminating water confinement with contactless heating, a few very recent bio-inspired designs, confining a thin water layer (~mm thickness) within a 3D printed conic structure[24] or hydrophobic porous absorber[17], demonstrated significant enhancement in salt rejection Despite these efforts, a pathway toward simultaneous thermal localization and salt rejection in a simple evaporator has remained elusive because the mechanics of salt transport during solar evaporation are not well-understood[8,20,26]. As a practical consideration, it is essential to develop a simple solar evaporator with fewer material restrictions and lower cost[8,20]

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