Exergy-efficient boundary and design guidelines for atmospheric water harvesters with nano-porous sorbents

Abstract

Atmospheric water harvesting (AWH) is now anticipated as a prospective solution to the current shortage of fresh water. The prevailing cooling-based (CB) strategy is proven inefficient in arid region where fresh water supply is more urgent. Based on the humidity enrichment nature of nano-porous sorbents, sorption-based AWH is proposed to fill this niche. Preliminary studies in this field focus mainly on system implementation under low-humidity conditions, rather than energy-conservation design and optimization due to the lack of relevant guidelines, leading to several energy-intensive prototypes and a misconception that the sorption-based technology is inherently energy-consuming. To break former stereotype, this work proposes a robust method of energy assessment, identifies clear scope of application for AWH technologies, as well as outlines materials and operational parameters choice guidelines. It is concluded that active sorption-based AWHs are not only the supplement of the cooling-based AWHs under arid weather, but also the efficient substitution except in tropical coastal region. Unexpectedly, the employment of adsorption cooling source can largely reduce the energy burden and weaken the demand for materials, while the one in condensation side imposes subtle influence. Considering the diurnal or seasonal weather variation in most areas, traditional sorbents with moderate stepwise position or linear isotherms might outperform the prevailing materials with early stepwise position. These findings provide new insights into the design of AWH technologies.