Understanding the Limits of Coupled Condensation and Desorption in Sorption-Based Atmospheric Water Harvesting (SAWH) Devices

Abstract

Access to clean water is a serious challenge around the world, with almost 2/3 of the global population experiencing water scarcity at some point during the year, especially in dry regions. One solution to this problem is sorbent-based atmospheric water harvesting (SAWH) due to its ability to produce drinking water in a range of environments, including at low humidity. SAWH device operation is composed of adsorption and desorption phases. During adsorption, moist air flows into the device and is adsorbed onto the sorbent bed. This is followed by the desorption phase during which the sorbent is heated to desorb the water as vapor, which is then transported to a colder condenser surface on which it is condensed as liquid water. Finally, the condensed water can be collected outside the device. However, current state-of-the-art SAWH devices are inefficient, with less than 70% of their adsorbed water being collected. This means the adsorbed water is either not condensed or condensed but not collected. This work discusses the impact of the coupling between desorption and condensation on the efficiency of SAWH devices. In general, SAWH systems can suffer from three scenarios of inefficient desorption-condensation: flux-limited, when the desorption rate in the device is insufficient to fully utilize the condenser’s condensation capacity; transport-limited, when the time scale of the vapor transport from the sorbent bed to the condenser is slow compared to the desorption operation time; and condenser-limited, when the condenser has a poor thermal design compared to the vapor flux. We developed a system-level model of a SAWH device to inform design strategies to mitigate these three bottlenecks and optimize device performance. Additionally, we quantified hydrocarbons, common airborne contaminants, as a mechanism for slowing water collection. Experimental findings are used to develop a model for the impact of airborne hydrocarbon adsorption on surface wettability and water retention for six metals commonly used as condenser materials. The findings from these models can inform design recommendations for SAWH devices as well as various other industrial applications in which water condenses on metal surfaces such as refrigeration and power generation. Future work will focus on continued experimental validation of the models.