## 3 AWG Evaluation Tools Short Review
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Name and Reference** & **Expression** & **Measure Unit** & **Definition** & **Notes** \\ \hline Recovery Ratio (RR) [25] & \(\frac{\frac{\chi_{i}-\chi_{e}}{\chi_{i}}}{\chi_{i}}\) (3) & [-] & Percentage of collected water on the whole water content of the treated air & This indicator is seldom used for industrial AWGs. The expression was corrected and rearranged by the authors on the basis of the definition found in [24]. The higher the percentage, the greater the water collection efficacy of the air treatment process. & This indicator was born to measure the efficiency of active AWGs. Nevertheless, the cooling energy includes only the sum of sensible and latent heat required to achieve atmospheric water condensation and does not fully account for the total energy consumption of the examined AWG. The water density is assumed equal to 1000 kg/m\({}^{3}\). The lower the value, the greater the AVG effectiveness in water extraction. \\ \hline Specific Energy Consumption (SEC) & \(\frac{\mathcal{Q}_{c}}{\omega}\) (4) & [kWh/kg] or [kWh/L] & Cooling energy required for the condensation of a unit of water mass or volume (\(w\)) & Although normally used for the evaluation of passive AWG systems, it can be applied to all technologies. The higher the value, the greater the density of water extraction, and thus, the compactness of the system. \\ \hline Specific Water Production (SWP) [25] & \(\frac{\varpi}{d-A}\) (5) & [kg/(day m\({}^{2}\))] & Prediction per time unit of a day (\(d\)) per unit of collector area (\(A\)) & This tool is used to characterise the yielding of AWGs based on desiccant materials. \\ \hline Specific, per Mass & & & This indicator is the most widely used, although it is improperly referred to as SEC, to assess the energy efficiency of active AWGs. \\ \hline \begin{tabular}{c} Specific Water \\ Production (SWP) [25] \\ \end{tabular} & \(\frac{\varpi}{d-A}\) (5) & [kg/(day kg)] & \begin{tabular}{c} Specific water mass \\ production per time unit of a day per unit mass of \\ adsorbent material (\(m_{a}\)) \\ \end{tabular} & This tool is used to characterise the yielding of AWGs based on desiccant materials. \\ \hline \hline \end{tabular} \begin{tabular}{c} Unit Power \\ Consumption (UPC) \\ \end{tabular} & \(\frac{\varpi}{d}\) (7) & [kWh/kg] or [kWh/L] & \
\begin{tabular}{c} AWG energy whole \\ consumption for the \\ production of one unit of \\ water mass or volume \\ \end{tabular} &
\begin{tabular}{c} This indicator is the most widely used, although it is improperly referred to as SEC, to assess the energy efficiency of active AWGs. \\ \end{tabular} \\ \hline Water Energy Transformation (WET) [21] & \(\frac{\varpi\cdot Q_{total}}{\varpi}\) (8) & [-] & The indicator is a dimensionless ratio of two energies, representing the relationship between the useful effect and the energy required to achieve it. It can be applied to both passive and active AWG machines. _en_ represents the required energy, which can be expressed in terms of primary energy to allow a comparison of passive and active systems. The higher the value, the greater the efficiency of the system. \\ \hline \end{tabular}
\end{table} 
Table 1: _Cont._GEI and WET will be discussed further in the next sub-section, while a short analysis of the other tools, presented in the table above, is provided below. MHI is one of the most cited metrics in the AWG research works and was developed to assess the suitability of climates for atmospheric water extraction through active cooling. It can be used \"ex ante\" when selecting a specific water extraction technology, considering only the climatic conditions of the installation site. The originators of the MHI have identified a value of 0.3 as a threshold to determine whether VCRC AWGs are effective. High values of MHI denote humid and hot climates, where atmospheric water extraction is more feasible with VCRC AWGs. Nevertheless, such an index is not suitable for assessing the effectiveness of advanced integrated AWG machines [34].