Adsorption-Based Atmospheric Water Harvesting: Impact of Material and Component Properties on System-Level Performance

ConspectusAtmospheric water harvesting (AWH) is the capture and collection of water that is present in the air either as vapor or small water droplets. AWH has been recognized as a method for decentralized water production, especially in areas where liquid water is physically scarce, or the infrastructure required to bring water from other locations is unreliable or infeasible. The main methods of AWH are fog harvesting, dewing, and utilizing sorbent materials to collect vapor from the air. In this paper, we first distinguish between the geographic/climatic operating regimes of fog harvesting, dewing, and sorbent-based approaches based on temperature and relative humidity (RH). Because utilizing sorbents has the potential to be more widely applicable to areas which are also facing water scarcity, we focus our discussion on this approach. We discuss sorbent materials which have been developed for AWH and the material properties which affect system-level performance. Much of the recent materials development has focused on a single material metric, equilibrium vapor uptake in the material (kg of water uptake per kg of dry adsorbent), as found from the adsorption isotherm. This equilibrium property alone, however, is not a good indicator of the actual performance of the AWH system. Understanding material properties which affect heat and mass transport are equally important in the development of materials and components for AWH, because resistances associated with heat and mass transport in the bulk material dramatically change the system performance. We focus our discussion on modeling a solar thermal-driven system. Performance of a solar-driven AWH system can be characterized by different metrics, including L of water per m² device per day or L of water per kg adsorbent per day. The former metric is especially important for systems driven by low-grade heat sources because the low power density of these sources makes this technology land area intensive. In either case, it is important to include rates in the performance metric to capture the effects of heat and mass transport in the system. We discuss our previously developed modeling framework which can predict the performance of a sorbent material packed into a porous matrix. This model connects mass transport across length scales, considering diffusion both inside a single crystal as well as macroscale geometric parameters, such as the thickness of a composite adsorbent layer. For a simple solar thermal-driven adsorption-based AWH system, we show how this model can be used to optimize the system. Finally, we discuss strategies which have been used to improve heat and mass transport in the design of adsorption systems and the potential for adsorption-based AWH systems for decentralized water supplies.