## 6 Economic Evaluations  
As described in the previous sections, the integrated AWG machine is able to give an important contribution to energy savings of existing HVAC plants in hot climatic conditions. Nevertheless, it is interesting to understand whether the investment, comprising integrated machine cost, installation and maintenance, has a positive return in an acceptable period. The costs of the entire installation, included the integrated machine, piping, pumps, links, manpower and transport, were equal to EUR 292,170. The maintenance and the consumables, including the water treatment unit, were calculated equal to 12,000 EUR/year. The revenue of the investment is the expected money saving related to the avoided consumption of bottled water, LPG, electricity employed for domestic water heating and electricity employed for primary air cooling. On the basis of the previous results, taking into account costs and prices of each voice and the currency conversion of 0.236 EUR/AED, the following Table 4 can be edited. In Table 4, the Pay Back Time (PBT) was calculated without actualisation, simply dividing the total investment by the yearly net revenue. In order to provide a more precise calculation, it was decided to carry out the Net Present Value (NPV) and the actualised PBT evaluation. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Savings** & & **Unitary Costs** & **Yearly Savings (EUR/Year)** \\ \hline LPG (L/year) & 54,426.1 & 0.4484 EUR/L & 24,405 \\ \hline Electricity due to boilers (kWh/year) & 634,509.5 & & 65,887 \\ \hline Electricity related to air & 190,259.5 & & 19,757 \\ \hline Bottled water (L/year) & 570,614.8 & 0.165 EUR/L & 94,151 \\ \hline Costs & & Unitary Costs & Yearly costs (EUR/year) \\ \hline Integrated AWG & & & \\ machine electricity consumption (kWh/year) & 331,775.2 & 0.10384 EUR/kWh & 34,452 \\ \hline Maintenance and & & & \\ consumables (EUR/year) & 12,000 & - & 12,000 \\ \hline Net revenue (EUR/year) & & 157,748 \\ \hline Starting investment (EUR) & & 292,163 \\ \hline Pay Back Time (years) & & 1.85 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Savings, costs and net revenue of the investment related to the integrated AWG machine. The following well-known equation describes the NPV:  
\[\text{NPV }(i,N)=\sum_{\tau=0}^{N}\frac{Cu_{\tau}}{(1+i)^{\tau}}+\frac{V(N)}{(1+i)^{ N}} \tag{5}\]  
\(N\) is the time horizon and was set, in this case, equal to 10 years, even if, thanks to the robustness of the components and of the employed technology, the expected useful life of the integrated machine is 20 years. The limited time horizon is a mean to provide safer results. The time period, \(\tau\), is equal to 1 year. \(Cu_{\tau}\) is the net cash flow. At year zero, it is negative and equal to the entire investment cost: EUR 292,170. As described in previous sections, the integrated machine use is the mean of the savings given by the avoided purchase of bottled water, electricity for boiler and air conditioning, and LPG for domestic water heating. The sum of those savings gives a positive cash inflow. The sum of the maintenance operations, consumable items and consumed electricity for the machine work costs gives a cash outflow that is negative. In order to provide safer results, the cash outflow was also charged, yearly, with 0.5% of the entire investment amount, equal to EUR 1461, to take into account possible machine component failures and replacement costs. Table 5 shows the cash flows. The actualisation rate, \(i\), is the sum of inflation and discount rate. In this case, the discount rate was set equal to 4.5%, the value of the Emirates Bond rate. It was chosen as an investment to be compared with, because it requires a minimum investment of the same magnitude order as that related to the integrated machine, and it has a confrontable time horizon (12 years instead of 10) [49]. The inflation rate chosen for Dubai is 2.83% for 2023 and 2% for the next 9 years, in compliance, for the first 5 years, with the statistical precision taken from [50].