Thermodynamic modeling and design of high-performance adsorption-based atmospheric water harvesting devices

Abstract

Water scarcity is a grand global challenge since more than two-thirds of world’s population is experiencing water shortage. Atmospheric water harvesting (AWH) addresses this challenge by enabling decentralized freshwater supply in water-stressed and infrastructure-limited areas. Adsorption-based AWH, in particular, overcomes the climate limitations of conventional AWH technologies and has the potential to further expand clean water access to extremely arid regions. Despite innovations in adsorbent materials, however, fundamental understanding of the physical processes involved in the AWH cycle and how material properties impact the theoretical limits of AWH are lacking.

In this thesis, we develop a generalized thermodynamic framework to elucidate the interplay between adsorbent properties and operating conditions for optimal AWH performance. Our analysis considers the temperature-dependence of adsorption, which is critical but has been largely overlooked in past work. Using metal-organic framework (MOF) as an example, we show that the peak energy efficiencies of single-stage and dual-stage AWH devices, after considering temperature-dependent adsorption, increased by 30% and 100% compared with previous work. Moreover, in contrast to common understanding, we show that the adsorption enthalpy of MOFs can also be optimized and further improve the peak energy efficiency by 40%.

To guide the practical design of next-generation adsorption-based AWH devices, we also perform initial modeling and characterization of select subcomponents for enhanced device performance. For atmospheric air delivery, we show that both the Dyson V9 motor fan and miniature drone propellers are powerful and compact solutions. However, after taking the power consumption into account, we identify the Noctua industrial DC fan as the best candidate overall for air supply. In addition, we show that significant heat sink enhancement is needed to maintain the condenser at close to ambient temperature and sustain the high flux of vapor condensation prescribed by high water productivity. This work bridges important knowledge gaps between adsorbent materials development and device design, providing insights toward high-performance adsorption-based AWH technologies.