## 3 AWG Evaluation Tools Short Review
#### WET and GEI  
Among the aforementioned indicators and indices, WET and GEI play a particularly important role. The WET indicator was designed to assess the efficiency of AWGs regardless of the technology used in the system. It was developed by the current authors in order to address some drawbacks associated with SEC and UPC [21]. As previously mentioned, SEC neglects the energy consumption of important parts of the process. Conversely, UPC takes into account all the energy consumptions, including that of the water treatment unit, but disregards the resulting water quality. When using UPC, the achieved water quality must be identical for all the examined systems to allow a fair comparison. When it is not possible to ensure that the AWGs under evaluation achieve the same water quality, which is the most common situation, the water treatment unit consumption should be excluded from the calculation, as specified by the WET indicator. This latter indicator converts the useful effect of the water production in energy terms, referred to as water condensation energy, \(Q_{w}\). Such a calculation is enabled by the specific heat of condensation, \(Q_{cond}\), whose value can be approximated to 2460 kJ/kg (an acceptable value for the largest part of the real-world AWGs), or analytically derived with a temperature-dependent equation [21]. Specifically, \(Q_{w}\) can be written as:  
\[Q_{w}=w\cdot Q_{cond} \tag{12}\]  
Thus WET is a ratio between two energies: the desired useful effect, divided by the whole energy required by the process to obtain it. The indicator shares the same architecture and perspective of the Coefficient of Performance (COP) [39] and of the Energy Efficiency Ratio (EER) [40]. The formulation is dimensionless and in line with the concept of efficiency, as it is the ratio between the benefit and the cost. Due to this format, the indicator can be directly used in the GEI equation. Regarding this latter index, it is important to highlight that it was developed to address the lack of evaluation tools for assessing the efficiency of multipurpose HVAC machines, in particular when such machines are able to produce water. The observation underlying the formulation can be summarised as follows: an integrated machine, which can produce \(n\) useful effects with an energy consumption equal to \(en\), should be \(n\) times more efficient than a system composed of \(n\) single machines each consuming \(en\) while producing only one useful effect. The index does not include arbitrary weights, thereby avoiding one of the main issues concerning index formulation [41]. Equation (1) can be used to compare different plant configurations, encompassing combinations of integrated machines or non-integrated ones. When more than one energy source is used in the plant, the expression (1) of GEI (Table 1) can be expressed in terms of primary energy by means of conversion factors [42], \(f_{i}\), employing, for example, the values proposed by the standard EN ISO 5200-1 [43]. \[\text{GEI}_{\text{primary}}=Id\cdot\sum_{i=1}^{Id}\frac{E_{i}}{f_{i}} \tag{13}\]  
Due to its formulation, GEI provides, in the form of a single increasing number, an easy metric to account for the influence of integration in assessing plant efficiency. The discussion about its usefulness and its adherence to analytic efficiency evaluations was presented in [24]. In the current paper, it is worth mentioning that the index is suitable for evaluating homogenous plant configurations, whether integrated or non-integrated. However, it is not directly applicable when the required useful effects are partially covered by a multipurpose machine alongside single-purpose machines, or when a multipurpose machine collaborates with other equipment to achieve all the required useful effects. In many real-life cases, such non-homogenous configurations are employed, in particular when retrofitting actions are carried out. It is common to find new equipment incorporating integrated solutions.