## 3 Results and Discussion
### Economic Optimization
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Zone A—Heraklion**} \\ \hline
**Collector type** & **A\({}_{\text{col}}\) (m\({}^{2}\))** & **\(\beta_{\text{opt}}\) (\({}^{\circ}\))** & **F** & **LCC (c)** & **SPP (years)** \\ \hline Simple FPC & 40 & 50 & 89.37\% & 12,383 & 2.85 \\ Advanced FPC & 35 & 50 & 92.55\% & 11,088 & 2.78 \\ ETC & 30 & 50 & 91.35\% & 13,480 & 3.49 \\ \hline \hline \end{tabular}
\begin{tabular}{c c c c c c} \hline
**Collector type** & **A\({}_{\text{col}}\) (m\({}^{2}\))** & **\(\beta_{\text{opt}}\) (\({}^{\circ}\))** & **F** & **LCC (c)** & **SPP (years)** \\ \hline Simple FPC & 40 & 45 & 85.77\% & 14,737 & 2.63 \\ Advanced FPC & 40 & 50 & 92.05\% & 12,875 & 2.83 \\ ETC & 35 & 50 & 91.50\% & 15,518 & 3.59 \\ \hline \hline \end{tabular}
\begin{tabular}{c c c c c} \hline
**Zone C— Thessaloniki** & \\ \hline
**Collector type** & **A\({}_{\text{col}}\) (m\({}^{2}\))** & **\(\beta_{\text{opt}}\) (\({}^{\circ}\))** & **F** & **LCC (c)** & **SPP (years)** \\ \hline Simple FPC & 40 & 50 & 77.97\% & 19,405 & 2.68 \\ Advanced FPC & 40 & 50 & 85.09\% & 16,855 & 2.83 \\ ETC & 40 & 50 & 87.98\% & 19,329 & 3.95 \\ \hline \hline \end{tabular}
\begin{tabular}{c c c c c} \hline
**Zone D— Kastoria** & \\ \hline
**Collector type** & **A\({}_{\text{col}}\) (m\({}^{2}\))** & **\(\beta_{\text{opt}}\) (\({}^{\circ}\))** & **F** & **LCC (c)** & **SPP (years)** \\ \hline Simple FPC & 40 & 50 & 77.01\% & 20,784 & 2.53 \\ Advanced FPC & 40 & 50 & 84.61\% & 17,692 & 2.65 \\ ETC & 40 & 50 & 87.41\% & 20,110 & 3.71 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Summary of the optimum designs according to the maximization of the life cycle cost index. Figure 23: Life cycle cost for different locations with the optimum collector (Advanced FPC). subsection in order to take into consideration both the SPP minimization and the yearly solar coverage maximization. Another interesting conclusion is that the SPP takes relatively low values in all the cases. More specifically, the SPP is lower than 4.5 years in all the cases; a valuable result that clearly shows that the investment in solar thermal systems is a viable one. The high electricity price, which is considered at 0.20 \(\upcels\)/kWh in this work, is the main reason for the low SPP values. Therefore, this study comes to prove, in a clear and accurate way, that the recent increases in electricity prices make the exploitation of solar energy vital for the sustainability and viability of future energy systems. Figure 24: Simple payback period for different collecting areas and collector types for Heraklion. Figure 25: Simple payback period for different collecting areas and collector types for Athens. Taking into account the optimal choices according to the LCC minimization criterion (see Table 7), the SSP for Heraklion is 2.85, 2.78, and 3.49 years for the simple FPC, the advanced FPC, and the ETC, respectively. For the same collector type sequence, the SPP is 2.63, 2.83, and 3.59 years, respectively, for Athens, 2.68, 2.83, and 3.95 years, respectively, for Thessaloniki, while 2.53, 2.65, and 3.71 years, respectively, for Kastoria. The conclusion is that the ETC leads to higher SPP due to the higher investment cost compared to the other collectors.