Salt rejection/discharge strategies of a reverse-distillation device with a water layer for sustainable solar desalination: From two dimensions to three dimensions

The recently proposed solar-driven reverse-distillation device with a water layer has shown promising potential in solar energy conversion efficiency and impactful advantages in salt rejection by constructing a rapid brine flow driven by gravity/density differences. However, a lack of profound understanding and explanation of the salt-rejection mechanism and accurate salt-discharge regulation strategies may hinder the further improvement of reverse-distillation devices. In this paper, we thoroughly discuss and analyze the mechanism of how gravity affects the internal brine flow and determines the salt rejection/discharge performance while providing comprehensive salt-discharge regulation strategies for feed brine with a wide range of salinities. The results show that only a 4° inclination angle enables the two-dimensional (2D) device with a 1-mm-thick water layer in the dissolution mode to dispose 3.5 wt% brine for 8 h without salt saturation, while the value is also merely 10° for a three-dimensional (3D) device, indicating the realization of the brine flow driven by a small gravity component and demonstrating a strong adaptability for the device. In the salt discharge mode, the ratio of the discharge flow rate to the evaporation rate determines the salt saturation risk. We discuss the salinity distribution of the evaporation surface at varied feed-brine salinities and discharge flow rates, providing a theoretical basis for precisely matching the thermal characteristics and enabling improved design for efficient and sustainable solar-distillation devices. Finally, we discuss the scalability of salt rejection and propose a scheme for scalable water production, which may offer important support for further realizing large-scale sustainable solar desalination by using the reverse-distillation device with a water layer.