Novel solar evaporator with closely-stacked reverse U-shaped hydrogel tubes for long-term stable evaporation with excellent salt resistance

The effective long-term utilization of the solar desalination technology requires stable evaporation rates and the ability to prevent salt crystallization. To address these challenges, this paper develops a novel 3D stacked hydrogel solar evaporator using reduced graphene oxide (rGO), sodium alginate (SA), and polyvinyl alcohol (PVA) to create hollow hydrogel tubes having multiple capillary structures. This evaporator with closely stacked reverse U-shaped hydrogel tubes has an average evaporation rate of 1.51 kg m−2 h−1 for pure water evaporation and a solar evaporation efficiency of 93.65%, due to the presence of hydrophilic groups and the unique 3D structure. The simulated experiment demonstrates that the evaporation rates of different locations on the surface of the three-dimensional evaporator are non-uniform. This is due to the joint impact of the water supply in the hydrogel tubes and the surrounded air humidity fields. The evaporator maintains a stable evaporation rate greater than 1.31 kg m−2 h−1 for 12 h of light per day and for seven consecutive days in simulated seawater, with no formation of crystalline salt. The experimental and finite element simulation results show that the salt concentration at the evaporation interface increases to 4.54 wt% after operating the evaporator in a 3.5 wt% NaCl solution for a certain period. Due to the effective water supply and structural characteristics, the highly concentrated brine at the top can flow back to the water body along the concentration gradient, which prevents the salt crystallization and ensures the stable operation of the evaporator. This study designs a stable and efficient 3D solar evaporator for seawater desalination, which supports the theoretical application for addressing freshwater scarcity.