# Techno-Economic Analysis of a Solar Thermal Plant for Large-Scale Water Pasteurization
## 4 Cost Estimation Model
### Economic Optimization Scenarios  
Using the lumped-component model of the plant implemented in Simulink(r), an evaluation of the natural gas and electricity consumption during the years has been carried out for different combinations of solar multiple and TES duration (see Table 2 for an overview of the explored parameters for the plant configuration). In all the analyzed scenarios, a pasteurization temperature of \(75\,^{\circ}\)C and an expected lifetime of the plant equal to \(20\) years have been considered. The unit cost of water purification (expressed as EUR-cent m\({}^{-3}\)) obtained is shown in Figure 6: four different capacities of the plant have been analyzed, namely (a) \(50\) L s\({}^{-1}\), (b) \(100\) L s\({}^{-1}\), (c) \(250\) L s\({}^{-1}\) and (d) \(500\) L s\({}^{-1}\). In these calculations a \(30\%\) equity to \(70\%\) debt (i.e., \(WACC=2.02\%\)) mix has been considered, and the prices of the natural gas and electricity (including taxes) have been taken from Ref. [48]. The optimal configuration is found for \(12\) hTES duration in all the considered cases; whereas, the optimal solar multiple varies depending on the size of the plant (see also Figure 7a). The minimum cost among all the explored cases has been found to be \(\approx 25\) EUR-cent m\({}^{-3}\), and is obtained for the \(500\) L s\({}^{-1}\) plant and SM = \(0.75\). In general, when the solar field is oversized, an increase of the TES duration allows to reduce the costs of water purification. Note that the reduction of the water treatment cost with increasing sizes of the thermal storage system is generally more effective close to the optimal value of the solar multiple.  
Clearly, the expected lifetime of the facility is an important parameter for assessing the depreciation rates. It is generally assumed that the useful life of a solar system is comprised between 20 and 30 years [44; 45; 54]. However, it is important to note that a plant is not only subject to physical degradation, but can also be replaced for different reasons, such as technical or commercial obsolescence. In fact, considering the development of new or unconventional technologies for water purification, the facility might be supplanted before the planned schedule. An analysis of the effect of lifetime on the 250 L s\({}^{-1}\) plant is shown in Figure 7b, where the unit cost of water purification is reported for lifetimes between 10 and 30 years (the optimal duration of the thermal energy storage is considered, that is 12 h in all cases). As the figure shows, the unit costs of water treatment rises as the lifetime of the facility decreases, since the depreciation rates become higher. In addition, for a useful lifetime equal or lower than 10 years, the curve of unit cost does not present a minimum for any SM, meaning that the most convenient solution is to carry out the treatment without the solar system. On the other hand, if the useful lifetime is higher than 15 years, the solar system allows to reduce the  
\begin{table}
\begin{tabular}{l c c} \hline
**Parameter** & **Range** & **Units** \\ \hline Plant water treatment capacity & 50 \(\div\) 500 & L s\({}^{-1}\) \\ Solar Multiple (SM) of the plant & 0 \(\div\) 3 & - \\ Thermal Energy Storage (TES) duration & 0 \(\div\) 12 & hours \\ \hline \end{tabular}
\end{table}
Table 2: Overview of the explored parameters for the plant configuration.  
Figure 6: Unit cost of water pasteurization (expressed as EUR-cent per cubic meter of treated water) as a function of the solar multiple and thermal energy storage duration for different treatment capacities of the plant: (**a**) 50 L s\({}^{-1}\); (**b**) 100 L s\({}^{-1}\); (**c**) 250 L s\({}^{-1}\); (**d**) 500 L s\({}^{-1}\). Calculations consider 20 years as expected plant lifetime.  
unit cost, and the most advantageous size is increasingly larger (from \(\text{SM}=1.00\) when the lifetime is 15 years, to \(\text{SM}=1.75\) when the useful lifetime is 30 years).