Peering into thermodynamic restrictions on adsorption-based atmospheric water harvesting

Atmospheric water harvesting (AWH), extracting water from the air, provides a promising solution to quench the world’s thirst. With energy-saving, portability and scalability features, adsorption-based AWH has had overwhelming attention from scientific and industrial communities. In recent years, enormous efforts have been made, while the lack of guidance from a thermodynamic viewpoint has led to lower thermal efficiency and dissensus with expectations. Herein, thermodynamic insights are presented to reveal the advantages of adsorption-based AWH, and the thermodynamic limits of various sorbents under different climate conditions are revealed. The results indicated that sorbents with stepwise position at high pressure have higher thermal efficiency, while the lifting thermal efficiency was compatible with sorbent’s practical suitability. Particularly, the cooling source-introduced sorption-based AWH was proposed to extend the feasible zone using specific sorbents, opening the box of thought in traditional AWH which suffers from limited sorbent suitability. Specifically, adsorption with cooling elevates the local humidity, resulting in an upgraded working capacity. When employing sorbents with an S-shaped isotherm, the impossibility of water harvesting was turned into a possibility when the ambient humidity was beyond the operational humidity range of sorbents. The energy consumption was accordingly calculated considering the heat pump or radiative cooling as an energy supplier. The results showed that the total energy consumption was increased due to the introduced cooling during adsorption, but the advantages in the low-temperature regeneration were more significant owing to employing sorbent with stepwise position at high pressure. The climate effect reveals that the performance on sorption was more sensitive to the humidity yet the desorption was affected by the ambient temperature dramatically. This paper would provide thermodynamic perspective for sorption-based AWH and pave a pathway for optimization of sorbent selection, device fabrication and working mode.