Conical solar-thermo-radiative evaporator for sustainable desalination and salt recovery

Desalination uses seawater to produce freshwater, offering potential solutions to address freshwater scarcity. However, it faces challenges such as high energy consumption, high-costs and mineral resources waste. Here, a conical solar-thermo-radiative evaporator is proposed to address the resource recovery in sustainable desalination. Three exciting concepts are integrated into such a novel structure: the photothermal converter captures full solar spectrum and converts it into infrared radiation that are more easily absorbed by seawater; the anti-gravity transport driven by passive capillarity and active negative-pressure evaporation enables continuous water supply through the conical porous materials; the marginal crystallization spatially isolates the salt crystallization from the water evaporation surface, promoting the salt recovery with assistance of gravity. Unlike normal evaporation structures, the radiative evaporation and natural evaporation collective works on both the inner and outer surfaces of the cone. This greatly promotes internal solute transportation and external vapor diffusion, leading to a high evaporation rate of 1.25 kg/(m2·h) with a solar efficiency of 89.9 % under one sun. We further show that stable desalination process can be sustained while simultaneously recovering salt during 40 h of non-stop operation, realizing spatial isolation of the salt crystallization from evaporation to harvesting freshwater and salt separately. This proof-of-concept work opens a new direction in development of solar evaporator with the zero-liquid-discharge ability for brine or wastewater treatment.