## 4 Results of the Optimizations
### Distributed Generation Solution Integrated with the Central Solar System  
The following results have been obtained by considering a superstructure which embeds all components included in the previous one, but a central system has been added, made up by an ICE, a BOI, the central storage and the solar field (Figure 10). Four optimizations have been performed: one economic optimization, one environmental optimization and two intermediate optimizations, obtained constraining the environmental objective function. Looking at the optimal configurations (Table 12) it can be noted that in all optimizations the solar field is adopted together with the central thermal storage, while the central BOI is not adopted because it is not economic to produce energy in the central unit and then transfer it to the sites, losing thermal energy through the DHN. Neither is the ICE is adopted, because the marginal cost of the heat results to be much higher when the electricity is sold to the grid (and therefore it has a low value) compared with the situation when the electricity is directly used by the users. Therefore, as the electric energy produced by the central ICE cannot be sent directly to the users, but can only be sold to the grid, the adoption of the central ICE is not economic. Figure 9: Optimal DHN of the 60% environmental optimum—distributed generation solution. In red the DHN pipes have been reported together with their size (kW). The tables report the size of the components installed. \begin{table}
\begin{tabular}{l c c c c} \hline  & **Environmental** & **70\% Env.** & **30\% Env.** & **Economic** \\ \cline{2-5}  & **Opt.** & **Opt.** & **Opt.** & **Opt.** \\ \hline DHN pipes [n°] & 14 & 8 & 7 & 7 \\ Central pipe size (kW) & 7500 & 6323 & 3579 & 1907 \\ ICE (kW) & 4920 & 1840 & 2380 & 2500 \\ MGT (kW) & 0 & 0 & 0 & 0 \\ BOI (kW) & 3480 & 3408 & 2730 & 2023 \\ ABS (kW) & 3570 & 1260 & 1155 & 1085 \\ HP (kW) & 3570 & 1120 & 1225 & 1155 \\ CC (kW) & 1584 & 1056 & 1053 & 1233 \\ PV panels (k\(W\)p) & 225 & 225 & 225 & 225 \\ ST panels (m\({}^{2}\)) & 0 & 0 & 0 & 0 \\ TS (kWh) & 0 & 0 & 2315 & 5134 \\ CS (kWh) & 0 & 0 & 0 & 0 \\ Central ICE (kW) & 0 & 0 & 0 & 0 \\ Central BOI (kW) & 0 & 0 & 0 & 0 \\ ST field (m\({}^{2}\)) & 27,736 & 23,585 & 19,013 & 8035 \\ Central TS (kWh) & 400,000 & 173,935 & 41,855 & 19,025 \\ \hline \end{tabular}
\end{table} 
Table 12: Optimal configurations of the distributed generation solutions Integrated with central solar unit. Figure 10: Superstructure of the distributed generation solution integrated with central solar unit. A difference to be highlighted concerning the previous section is the presence of the boilers to cover the thermal peaks, the size of which increases as we move towards the environmental solution. A separation in the DHN is still present in the economic and 30% environmental optimizations, forming two separated subsystems: sites 1-6, and sites 7-9, with the second group only connected to the central unit. Starting from the 70% environmental optimization, all users are connected to each other, whilst the storages installed in the local production units disappear, replaced by a larger central thermal storage, with lower heat loss. In parallel, the solar thermal field increases threefold compared with the economic optimum. The size of ABSs increases, too, while the ST panels at the user site are never adopted. Table 13 shows the economic and environmental results of the optimizations. The trends of the costs and of the emissions are similar to the ones observed in the previous optimizations. Comparing Table 10 with Table 13, it can be observed that the operating cost of the solutions integrated with the central solar system is lower (\(-6\%\)), while they are characterized by higher investment costs (+30%). It is worth noting that the Pareto front obtained with integrating the central solar system (Figure 11) dominates the other Pareto fronts, as all the solutions obtained allow lower annual costs together with lower annual emissions to be achieved.