Thermodynamic limits of atmospheric water harvesting with temperature-dependent adsorption

Abstract

Adsorption-based atmospheric water harvesting (AWH) has vast potential for addressing global water shortage. Despite innovations in adsorbent materials, fundamental understanding of the physical processes involved in the AWH cycle and how material properties impact the theoretical limits of AWH is lacking. Here, 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 largely been 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%, respectively, compared with previous studies. Moreover, in contrast to common understanding, we show that the adsorption enthalpy of MOFs can also be optimized to further improve the peak energy efficiency by 40%. This work bridges an important knowledge gap between adsorbent materials development and device design, providing insight toward high-performance adsorption-based AWH technologies.

All‐Day Multicyclic Atmospheric Water Harvesting Enabled by Polyelectrolyte Hydrogel with Hybrid Desorption Mode

“…[8] However, the limited adaptability of MOF to changes in environmental RH, along with stronger temperature dependence of the vapor sorption isotherm, the difficulty and expensive large-scale synthesis, as well as larger deterioration of heat and mass transfer after modularization have hindered the wider applications in practical scalable water harvesters. [9] Hygroscopic salt-embedded composite materials (HSCM) that are synthesized by the facile embedding process of the hygroscopic salt (e.g., lithium chloride (LiCl)) into the matrix, have been widely investigated in AWH systems, particularly for scaleup applications. [10] Such materials show advantages in low-cost fabrication, facile and scalable preparation methods, and strong adaptability over wide air humidity ranges (≈10-100%).…”

“…[ 8 ] However, the limited adaptability of MOF to changes in environmental RH, along with stronger temperature dependence of the vapor sorption isotherm, the difficulty and expensive large‐scale synthesis, as well as larger deterioration of heat and mass transfer after modularization have hindered the wider applications in practical scalable water harvesters. [ 9 ]…”

Addressing global water stress using desalination and atmospheric water harvesting: a thermodynamic and technoeconomic perspective

“…To find the LCOW of AWH systems in $ per m 3 , eqn (2) is used:where CAPEX i is the capital expenditure of the AWH technology (in $ per unit system size), CAPEX WS is the capital expenditure of water storage (in $ per m 3 of water storage), V WS,i is the volume of water storage needed for a given AWH technology (in m 3 of water storage per unit system size), CRF is the capital recovery factor or amortization factor, OPEX fix accounts for fixed operations and maintenance costs (in $ per unit system size per year), Yield i is the annual water produced (in m 3 of water per unit system size per year), and OPEX e is the variable operating costs associated with energy consumption (in $ per m 3 of water produced). The subscript i in eqn (2) corresponds to a particular AWH technology being analyzed (active cooling, passive cooling, or sorbent), with the system size measured in tons of refrigeration, m 2 of radiative cooling surface (assuming a TPX polymethylpentene sheet), and kg of sorbent (assuming MOF-303 30,34 ), respectively. All the input costs and other assumptions for this model are provided in Supplementary Note 1 (Table S1, ESI†).…”

“…Literature reports on AWH energy consumption are limited to thermodynamic analyses of different systems rather than real data under varying operating conditions – for example, Rao et al 24 and Kwan et al 33 evaluated the least work of separation for reversible AWH, and they found that AWH systems operate at a fraction of the reversible limit. Li et al 34 developed models for irreversible (semi-idealized) sorbent AWH in single and multi-stage configurations, and they found that its energy consumption is 67% higher than the enthalpy of vaporization of water. This is an important finding since other processes that rely on evaporative phase change such as thermal desalination are known to be prohibitively expensive and inefficient unless heat recovery is implemented.…”

“…The analysis framework is also used to (ii) determine the LCOW of three AWH technologies (Fig. 1c) – dewing from active cooling, 42 dewing from passive/radiative cooling, 32 and heat-driven sorption systems 26,34 – to pinpoint locations where each would be cost-effective. These results (for practical/irreversible AWH as well as hypothetical reversible AWH systems) are compared to the LCOW of seawater desalination ($1 per m 3 ), with energy costs added for transporting clean water to different inland locations (using geography data 43 and water transport costs 14 ).…”