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

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.

Critical aspects to enable viable solar-driven evaporative technologies for water treatment

“…It thus appears natural to take advantage of the prior long-standing know-how gained in the field of membranes for nanofiltration 29 . Interestingly, such approaches can be also combined with highly effective passive flow manipulation exploiting the Marangoni effect 20 and natural convection 30 to enhance salt rejection.…”

A reconfigurable and magnetically responsive assembly for dynamic solar steam generation

“…However, further improvement of evaporation performance has encountered bottlenecks 33 , 34 . The conventional evaporators are structurally fixed, incapable of disassembling and deforming, thus generally inducing the salt deposition and lowering the evaporation efficiency 35 – 38 , although different strategies have been tried including Janus structure 18 , 36 , localized crystallization 35 , contactless evaporation 37 , 39 , 40 , convective flow of water 38 , 41 , and wick-free water layer 42 . Most of them only involve the passive fluidic flow of water, the active circulation of water and salt ions has not been reported, which could deal with the problem of the slow diffusivity in passive convection 42 .…”

“…The conventional evaporators are structurally fixed, incapable of disassembling and deforming, thus generally inducing the salt deposition and lowering the evaporation efficiency 35 – 38 , although different strategies have been tried including Janus structure 18 , 36 , localized crystallization 35 , contactless evaporation 37 , 39 , 40 , convective flow of water 38 , 41 , and wick-free water layer 42 . Most of them only involve the passive fluidic flow of water, the active circulation of water and salt ions has not been reported, which could deal with the problem of the slow diffusivity in passive convection 42 . On the other hand, in these static evaporation systems, evaporation rate is largely restricted by the accumulated water vapor around the evaporation surface, which results from the insufficient water vapor diffusion (WVD) from evaporators to surroundings 43 , 44 .…”

Double-Sided Suspending Evaporator with Top Water Supply for Concurrent Solar Evaporation and Salt Harvesting

“…Employing waterproof thermal insulation layers reduces heat leak but forces water to go through the side walls to the evaporation surface center, , which can hardly satisfy the evaporation rate on a larger evaporation surface. Using a porous thermal insulation layer allows the water below to be directly transported to the evaporation surface, which solves the water supply issue for large evaporation surfaces, , but greatly suffers the heat loss problem through water channels. More importantly, when applied on seawater, salt would accumulate on the photothermal materials and gradually stop the system. − …”