Challenges and Strategies Toward Sustainable Atmospheric Water Harvesting

Abstract

Conspectus Facing the growing stress on freshwater supplies, harvesting water from the atmosphere via sorbents has garnered significant attention due to its broad applicability, regardless of geographic and hydraulic restrictions. In advancing the sustainable development, two critical aspects are the use of biomass-derived sorbents and solar energy. Biopolymers offer viable alternatives to petroleum-derived synthetic polymers, presenting opportunities for developing environmentally friendly AWH systems. Additionally, efficient capture and utilization of solar energy to drive water desorption are also critical to enhancing the sustainability of AWH. This account discusses the challenges and strategies in efficiently utilizing biomass and solar energy for sustainable AWH. We begin with the key challenges in these two domains. Transforming biomass into efficient sorbents necessitates molecular and structural engineering to achieve high water uptake capacities and rapid sorption–desorption kinetics. The inherently intermittent and relatively low power density of solar energy introduces additional energy challenges, demanding heat and mass transfer management to maximize sunlight utilization. We then highlight various strategies to mitigate these challenges and thus promote water yield in sustainable AWH systems. To endow biopolymer-based hydrogels with enhanced hygroscopicity and hydrability, extracting nanofibrils from biomass has proven effective via exposing more active sites (e.g., hydroxyl groups), significantly enhancing water uptake capacities. Chemical modifications, such as introducing zwitterionic groups, have also been demonstrated to promote polymer swelling and water capture. Structurally engineered biopolymer hydrogels further facilitate internal water diffusion, accelerating sorption and desorption. To address the energy challenges, developing thermoresponsive sorbents that release water under lower, solar-compatible temperatures is beneficial. Additionally, exploring advanced solar absorbers with high absorptivity and selective thermal emission properties can maximize solar energy capture and minimize radiative heat losses. Device-level designs that reduce radiative heat exchange and minimize view factors between sorbents and other components further contribute to efficient thermal management. Integrating heat recovery, such as reusing latent heat released from water condensation, can further improve the overall energy utilization efficiency. Efficient water transport is also essential for improving overall water yield. Employing low-dimensional sorbents like two-dimensional thin films and microgels can accelerate water diffusion within sorbents with reduced effective diffusion length. Alternatively, directional pores can be introduced in bulk sorbents to reduce tortuosity, which also benefits internal diffusion. Moreover, by introducing forced convection, the actively driven airflow can efficiently carry water vapor away from sorbent surfaces toward condenser, preventing local vapor accumulation and suppressing the humidity buildup that would otherwise inhibit further desorption. Finally we highlight future challenges and opportunities in promoting sustainable AWH.

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