## 1 Introduction  
Today, the water crisis affects billions people: two billion face high water stress, as reported by the United Nations' (UN) World Water Development report [1], while four billion suffer water scarcity for, at least, one month a year [2]. The UN quoted document depicts a situation that will rapidly worsen, as the yearly water demand, today evaluated at about 4400 billion cubic metres [3], is expected to increase, between 20 and 30%, by 2050. At the same time, climatic changes and water pollution are exacerbating the crisis, because:  
* 1.8 billion people already use contaminated water [4];
* water-related disasters between 2001 and 2018 were estimated to be accountable for around 74% of natural disasters, expected to increase in the next few years [5]. To face the issue, strong requirements are represented by better hydride resource management, wastewater treatments, water recycle and reuse practices [6]. Additionally, new fresh water sources, achievable and sustainable, are needed [7]. In such a context, in the last decades, air has been rediscovered as a water source. In ref. [8], the possibilities to employ atmospheric water harvesting in order to enhance the water supply resilience were explored. One idea, from this perspective, is the recovery of condensate coming from air conditioning systems. In ref. [9], a review of the potentialitiesof such an approach was presented. Nevertheless, water quality plays a fundamental role. In particular, water coming from air conditioning systems can be affected by heavy metals and microbiological pollution [10]; thus, its legal employment normally is confined to uses not related to humans, as underlined, for example, in Dubai regulations [11]. Moreover, air conditioning systems are usually designed to minimise condensation, as the latent heat is perceived as an energy waste, because the condensate is normally treated as a polluted liquid to be disposed of. In this perspective can be ranked the use of desiccants on the inlet environmental air and the vapour disposal into the environment, by means of the hydrophilic substances regenerative air stream [12]. A more dedicated way to extract water from the air is represented by Atmospheric/Air to Water Generator (AWG) systems, also known as Atmospheric Water Harvesting (AWH) systems. The research in this field has been particularly active in the last few years, and various processes of water extraction have been explored. For example, in ref. [13], the available technologies were presented, and the research gaps were underlined, while the review presented in ref. [14] focused on the most promising techniques for water extraction in arid zones. The work described in ref. [15] collected the findings concerning systems that employ a passive approach. In the current work, a brief description of the state-of-the-art AWGs is reported in paragraph 2. It is worth noting that one of the main research questions concerns the need for more field tests about real machines of medium-large sizes (from hundreds to thousands L/day) that, up to today, are comparatively few, as underlined in ref. [13]. The same lack of tests--in particular, for Middle East regions--was also underlined in ref. [10]. As a matter of fact, it is possible to find some data coming from tests about little size systems capable of producing some litres per day up to a few hundreds. For example, in the review of [10], concerning the Middle East, the maximum water extraction test, performed by AWGs, gave a production of about 232 L/day. In ref. [16], a prototype able to produce up to 8 L/day in arid conditions was described. In ref. [17], there was a comparison of different models of machines able to provide less than 20 L/day. Studies based on desiccant materials or on thermoelectric coolers are normally sized to produce few litres per day; thus, tests carried out on prototypes of such kinds of machines describe productions that go from a few grams to a few kilograms of water per day, as it is reported in refs. [18; 19].