## 2 Atmospheric Water Capturing: The Integrated Approach  
There are three main forms of atmospheric water: rain, fog, and air vapour content. Atmospheric water capturing can be pursued with various techniques, depending upon the aforementioned forms [25]. There are several classifications of such techniques, nevertheless, one of the most used is the distinction between passive and active methods. Active systems employ external electricity or other high-grade power sources, whereas passive systems use exclusively solar heating and natural thermal gradients to achieve atmospheric water capturing [26]. Passive techniques include fog capturing using net-framed devices, which facilitate the coalescence of tiny droplets; water harvesting through sorption-desorption cycles with desiccant materials and natural temperature gradients; and dew collection employing radiative panels, which exploit natural radiative cooling [16]. Active systems encompass atmospheric water condensation by means of air cooling under its dew point, provided with VCRC or thermoelectric cooling, the latter based on the Peltier effect. Active systems also include a mixed technique that combines VCRC with desiccant materials to enhance water harvesting [16]. Providing an exhaustive review of the various techniques falls outside the scope of the current research. In addition, in the last few years, there have been several review works on atmospheric water capture. Among them, one of the most recent [27] underlinesthat the technique based on VCRC, in comparison to the others, gives the highest yields in a comparatively small space. It is worth mentioning that an active approach such as this also enables the possibility of filtering the air flux entering the inlet section, avoiding, in this way, a significant source of water pollution [16]. Simultaneously, the technology is mature enough to be commercialised, and, in fact, it is the most widely exploited in the real market [28]. Nevertheless, the energy consumed is one of its major drawbacks, because such a technique is an active one and requires external electricity to work, thus, further studies focused on this topic are strongly recommended [27]. In such machines, the process of water extraction from the air is based on vapour condensation by means of heat exchange between a flux of environmental air and a refrigerant, which works in a thermodynamic reverse cycle. The working cycle is represented in Figure 1 and can be described as follows. A flux of environmental air is blown through a heat exchanger, the evaporator, by a fan. In this coil, part of the air thermal energy transfers to a refrigerant flux, which is evaporating well below the dew point temperature of environmental air. In such a way, the environmental air flux is cooled below its dew point, causing the condensation of part of its vapour content. The heat is discharged into the environment through a second coil, the condenser. Here, the refrigerant, compressed by the compressor to a temperature above that of the environment, condenses, exchanging heat with a second environmental air flux, which is moved by a second fan. Finally, a lamination valve (expansion valve) returns the refrigerant to the initial conditions of the cycle. In addition to the working cycle described above, it is important to underline that to achieve good water quality, avoiding inorganic and organic pollutants, AWG should always be equipped with air filters in the inlet section and an adequate water treatment unit to transform the collected condensate into highly pure liquid [16]. From Figure 1, it is clear that a traditional VCRC AWG requires energy to run the compressor and fans. It is worth noting that the very process of vapour condensation--regardless of the employed technique--requires external energy if significant quantities of water are needed (on the order of hundreds or thousands of litres per day) in a comparatively small space (this excludes fog nets, radiative panels, and some desiccant-based solutions).