## 2 AWG Technologies  
Water extraction from the air, up today, can be carried out principally by means of humidity condensation. Vapour contained in the breathable air is forced to change its state into liquid. Different techniques can be used in foggy climates, where fog nets permit water extraction from air directly collecting the fog aerosol by means of coalesce [25]. In the other cases, the humidity contained in air must be captured from the atmosphere and transformed toliquid by means of thermal and/or chemical-physical processes. Such an operation can be carried out by means of refrigeration of the air under its dew point. The condensate is then collected and treated in order to achieve the required quality. Vapour-harvesting processes can be classified into passive and active. The processes that do not need an artificial energy input are defined passive [8], and an example of such an approach is represented by the radiative cooling, such as that employed in the dew plant described in ref. [26], where panels with a particular emissivity and geometry are installed in the open air. The temperature gradient, caused by irradiation, between the environmental air and the panel surfaces, permits vapour condensation. The main advantage of such a process is that it does not require an artificial energy input. The disadvantages are that: it can be carried out only in particular climates, it strongly depends upon weather conditions and requires large spaces to collect meaningful water quantities, usually represented by a few litres a day for a squared metre [8]. In the active processes, there are comprised AWGs based on a thermodynamic reverse cycle. Those that employ a compressor represent the most diffused and scalable technology, as stated in ref. [13] and confirmed by consulting [27]. The working principle is the following. Environmental air is forced to flow into an evaporator coil, where a refrigerant at a low pressure and temperature is flowing as well. The heat is removed by the refrigerant, and the air is cooled under its dew point. Condensate is collected and the processed air is disposed into the environment. The refrigerant, compressed, passes through the condenser coil, where it exchanges heat with the external environment and returns, after the lamination valve, to the starting conditions of the cycle. The main advantages of such an approach are the comparative high yields (up to thousand L/day), the compactness (dimensions comparable to those of an air conditioner chiller) and the possibility to operate the machine in a wide temperature and relative humidity range (even when the dew point is low, with defrosting cycles). The main weak point is the energy consumption that increases when the dew point is under 4 \({}^{\circ}\)C [28]. The cooling effect can also be obtained using thermoelectric coolers [29], instead of the compressor. The main advantages of such an approach, in comparison to the compressor use, is the compactness of the devices, the absence of moving parts and the absence of any kind of refrigerant. The weak points are represented by a low efficiency thus, the energy consumption is higher than that related to compressor employment and by a difficult regulation and a low yield (few L/day) [30]. Another approach to water harvesting consists into increasing the vapour content in the air by means of materials able to adsorb, or absorb, part of the atmospheric humidity. Once such materials are saturated enough, they are forced to release the vapour into a controlled air volume or air flux, normally using a low temperature heat flux [31]. After that, the process continues with the air cooling that permits collecting liquid water. In the desiccant field, a very promising substance is a super moisture adsorbent gel [32], a material able to adsorb moisture and release a part of it, directly in the liquid phase, by means of low temperature heating.