1
Introduction

Water is the source of terrestrial life and the necessary condition for reproduction and the continuation of life. Despite oceans cover 65% of the Earth's surface and contain 97.2% of global water, large portions are saline and undrinkable. Human use is limited to fresh water from freshwater lakes, rivers, and groundwater systems, accounting for only 2.53% of the world's water. Of this, about 70% is locked in polar and alpine glaciers, which are difficult to extract. Currently, our primary access to water includes rivers, freshwater lakes, and shallow groundwater - about 0.3% of global fresh water and 0.0007% of total world water. Although the ocean accounts for a significant proportion of global water resources (mostly saline), most available water comes from ice caps, glaciers, groundwater, inland seas, and rivers: 1.8%, 0.9%, 0.02%, and 0.001%, respectively. Regionally, nine countries, including Brazil, Russia, Canada, China, the United States, Indonesia, India, Colombia, and Congo, hold more than 60% of the world's fresh water, but at the same time, nearly a fourth (about 80 countries) of the global population faces severe shortages. Globally, about 1.5 billion people, or one sixth of the planet, suffer from inadequate supplies of water, with an additional third of countries amount to a billion people experiencing this crisis.

Harvesting water from ambient air holds potential for freshwater provision. Given all indicators, 3.8 billion people experienced varying levels of water scarcity in 2005 [1]. This is expected to persist until population growth peaks in 2050 [2]. Surface rivers constitute approximately 40% of precipitation across global land [3]. Nevertheless, WHO's report forecasts a 20-30% surge in demand for water by 2050 [4]. Urbanization and climate change compound this issue, with half of the urban populace projected to encounter water shortages by 2050 (Figure 1.1) [5].

Researchers analyze the scarcity of water resources across two dimensions, supply vs. consumption, while also appraising the quality of the supplied water in correlation to factors such as demographics, agricultural productivity, and topographical layout. Of the 304 cities examined, an astonishing 273 (about 89.8%) were confronted with water security risks in 2015. Among these, 262 municipalities (or roughly 86.1%) faced quality risk, another 85 entities (about 27.9%) ventured into scarce risk, and an additional 74 municipal governments (about 24.3%) encountered dual risks (Figure 1.1). Moreover, 69 cities (around 22.7%) were marked as severely exposed to extreme water quality risk, and another 48 (or almost 15.8%) being severely encumbered by severe shortage of water scarcity risk, while 7 (about 2.3%) were grappling with both extremely high-water quality risk and severe water scarcity risk.

Figure 1.1 Global scarcity of water. Distribution of large cities in hydric stress regions (capitals exceeding 10 million inhabitants in 2016).

Source: [5] / Springer Nature / CC BY 4.0.

Strangely enough, among the evaluated eighteen megacities (i.e. those with population exceeding 10 million), an overwhelming 25 (around 92.6%) were affected by water security risks. Among these, 23 cities (about 85.2%) grappled with quality risks, an additional 10 (about 37.0%) were involved in scarce risk, and 8 (or around 29.6%) had to contend with dual risks. Notably, eleven of these major cities including São Paulo, Mumbai, and Dhaka were severely under threat of highly risky water quality, whereas five major cities (Delhi, Beijing, Los Angeles, Moscow, and Bangalore) were severely burdened by severe water scarcity risk, with Bangalore bearing the brunt of dual risks on both ends [6].

In terms of technology, desalination is recognized as an effective method to alleviate freshwater scarcity crisis. Broadly, desalination can be categorized into thermally driven (distillation) and membrane-based processes. Thermal seawater desalination remains the predominant method in the Middle East region [7]. However, membrane desalination methods exemplified by reverse osmosis (RO) have rapidly grown since the 1960s, and more seawater desalination plants opt for RO technology [8]. Through analysis and comparison, it can be found that one of the paramount challenges associated with RO technology is high energy consumption, which may escalate greenhouse gas emissions [9]. Despite extensive work researchers undertaking in integrating seawater desalination with renewable energy to address energy consumption issues, seawater desalination to obtain freshwater is impractical in inland regions distant from the ocean. Furthermore, even at small scales, seawater desalination technology not only necessitates accessible brackish water sources but also competent operators and maintenance personnel, which significantly restricts the application scope of seawater desalination. However, the geographical environment and climatic conditions of remote or inland areas, as well as issues concerning cost, energy consumption, secondary pollution, etc., impose limitations on the development of the technology [10]. Consequently, there is an urgent need to develop a promising technology to tackle situations in remote areas or areas where economic levels do not advocate centralized treatment and distribution networks [11].

As an immense renewable reservoir, the atmosphere contains approximately 12?900?km3 of water [12], sufficient to meet part of the water needs of households, agriculture, and industry. Even in remote regions and arid deserts, abundant water vapor is present; the water in the atmosphere is deemed an enormous renewable reservoir. Capturing water vapor from air not only saves significant labor and energy costs but is universally viable across vast regions of Earth. The water vapor in the air is relatively free of impurities and bacteria, and the water harvested from air can reach potable water standards without elaborate purification and sterilization processes. Unlike seawater desalination, when water is extracted from air, it hardly disrupts the water cycle and does not have any detrimental impact on nearby crucial water resources after transportation. Even in the driest desert regions, water vapor is widely dispersed in the form of molecules in the atmosphere. Atmospheric water harvesting (AWH) produces fresh water through the collection of moisture in the air, enabling sustainable water transport without substantial infrastructure and geography or hydrology restrictions [13]. A viable water heating technology should meet the following primary criteria: robust water harvesting capabilities, low energy consumption, affordability, stability, and negligible constraints imposed by the environment and climate [14]. Therefore, the development of efficient AWH technology paves the way to harness this portion of the atmospheric water resource and holds promise to address the freshwater shortage issue, particularly in geographically distant or rural regions where water transportation is costly [15].

1.1 Overview of Research on Atmospheric Water Collection

Traditional fog collectors are a simple yet sustainable system, implemented when a gauze material is exposed to cloud banks. Some droplets collide and accumulate on the gauze material, aggregating to form larger fog droplets, which are subsequently attracted by gravity and channeled into drainage pipes, ultimately reaching water tanks or distribution systems. They can be categorized as standard fog collectors (SFCs) and large fog collectors (LFCs) . SFCs typically find use in exploratory research for assessing the potential quantity of fog accumulated under specific conditions. LFCs primarily perform actual fog collection operations. If the performance of an SFC surpasses a certain threshold, it transitions seamlessly into an LFC. Since the mid-twentieth century, conventional fog collecting initiatives have enjoyed considerable success in Chile, subsequently being deployed in several parts across the world. These locales feature climates and geographies conducive to natural fog gathering predominantly arid tropical and subtropical regions with high altitude. In most nations, the prevalent fog collector preferred involves placing a Raschel grid vertically between two poles to harness moisture from fog [16]. The grid features a trihedral pattern, comprising fine filaments and pores at the millimeter level. Woven polyolefin Raschel nets are a favored capturing medium, treated with UV protection and possessing a shading coefficient of 35%. Eiffel, an innovative 3D fog collector, comprises two layers of Raschel meshes and ten meshes. It optimizes the collection of windblown fog parallel to a heat exchanger, achieving a collection efficiency of 281.2?L?day-1 - approximately 10 times that of standard full-sized heat exchangers. However, this ideal scenario for collecting fog is subject to strict constraints. Consequently, the emergence of "Harp" and "Diagonal Harp" fog processors has made significant strides beyond these limitations. Comprising a series of closely spaced vertical stainless steel wires, close to a fog net lacking horizontal lines, they capture fog originating from all directions, enhancing the precipitation rate of liquid droplets, averting blockages, thereby enhancing the performance of the collector [17].

However, some of traditional fog collectors merely harvest a portion of fog...