Real‐Life Solar‐Thermal Water Purification Exceeding 300% Efficiency Using 2D Planar Interfacial Evaporator

Abstract Solar‐thermal water purification is an environmentally friendly way to provide potable water. However, the available evaporator materials have inherent geometrical limitations leading to low practical evaporation efficiency in real‐life outdoor operation. Here, a 2D planar interfacial evaporator made of a nanostructured superwicking black metal (SWBM) surface is demonstrated that simultaneously tracks the sun, absorbs virtually all the incident solar energy, pulls water out of the reservoir onto the heated surface, and automatically rejects the salt at all‐time with or without sunlight thus can enable maintenance‐free and continuous seawater desalination with zero liquid discharge. Under 1‐sun solar irradiation, the SWBM evaporator achieves an evaporation rate of 2.59 ± 0.11 kg m − 2 h −1 in a closed system, which is ≈70% of its performance in an open system (3.69 ± 0.11 kg m − 2 h −1 ). When calculating evaporation efficiency using the bulk water evaporation enthalpy, a record value of 325% in an outdoor operation is achieved. The higher‐than‐ideal efficiency is due to a combined effect of low evaporation resistance, thin‐film heat transfer, and reduction in the vaporization enthalpy. The outdoor evaporation rate at 25° solar zenith even surpasses the highest measurements ever achieved in ideal laboratory conditions under one‐sun normal incidence. The system operates efficiently through all seasons at all solar positions, which is not possible for other existing solar evaporators. In the summer month of June, an outdoor water evaporation rate of 3.6 kg m −2 h −1 (244% efficiency) at noon and 2.45 kg m −2 h −1 (166% efficiency) during sunrise and sunset is achieved when the sun is on the horizon. The solar‐tracking effect of the SWBM panel leads to a 40%–65% higher performance in summers, while even higher at 100%–300% in winters as compared to having the same panel lying horizontally. The SWBM solar‐tracking device is used to desalinate water with a range of salt concentrations, and the purified water has the salt level much below the World Health Organization and Environmental Protection Agency standards for safe drinking.