Optimising desiccants for multicyclic atmospheric water generation: Review and comparison

Atmospheric water generators produce liquid water from humidity in the air. Hence, this technology provides a pathway to alleviate water scarcity. In contrast to conventional day-night monocyclic systems, multicyclic atmospheric water generators conduct multiple sorption and desorption cycles per day. The specific water production for multicyclic desiccant based atmospheric water generators primarily depends on the water sorption and desorption rates of the desiccant, as opposed to the uptake capacity. The mechanisms governing the equilibrium uptake capacity of desiccants and the interparticle diffusion rate of water vapour are well known, however, the mechanisms governing the intraparticle diffusion and sorption rate of water vapour within desiccants are not well summarised. In this review, methods for the enhancement of the intraparticle water vapour diffusion and macroscopic sorption rate are identified, including the effects of pore microstructure, surface hydrophilicity, and composites. Additionally, desiccants with the highest potential specific water production and lowest potential specific energy consumption are identified. To date, the polyamide 6-LiCl nanofibrous membrane demonstrates the highest ideal specific water production of 230 L.kg(-1).day(-1). The ideal specific energy consumption is similar between the investigated desiccants and primarily depends on the latent heat of sorption. Furthermore, the suitability of various empirical kinetic models for the investigated desiccants is discussed. The variable order model provides a better fit to sorption and desorption kinetic data than the commonly used linear driving force model.