Solar steam water generation for water purification and fog collection for water production are two advanced water supply technologies combined into one system by California Institute of Technology engineers.

The key to the multifunctional device is a micro-architected polyvinyl alcohol/polypyrrole composite hydrogel membrane into which a specific nanoscale pattern is etched. The hydrophilic surface contains an array of tiny structures inspired by cactus spines and is designed to absorb sunlight to heat water trapped beneath it. The steam generated condenses onto a transparent cover, where it can be collected. At night, the transparent cover folds up and the hydrogel membrane is exposed to humid air to capture fog.

Schematic of the hydrogel membrane with micro-topologies that is capable of 24-hour freshwater harvesting. Source: Ye Shi et al./California Institute of Technology

Tests conducted with material sample sizes of 55 cm² to 125 cm² effectively collected about 125 ml of water from solar steam during the day and harvested 35 ml of water from fog at night. The researchers also demonstrated that the solar steam generation and fog harvesting functions could be coupled in one floating system, and a daily yield of 34 liters/m² was achieved by a rooftop system designed with the technology described in Nature Communications.