## 3 Results and Discussion
### Economic Optimization  
The economic analysis of this work is conducted only for the cases with optimum tilt angles because in these cases, the solar coverage is maximized, and the auxiliary energy consumption is minimized. So, the operational cost of the system is minimized. Moreover, the placement of the solar collectors at different angles does not lead to different costs, thus there is not any reason to take into account all the tilt angles in the economic analysis. Practically, the optimum economic choices would be for cases with optimum title angles. Therefore, the examined parameters in the economic analysis are the collecting area and the collector type. Figures 19-22 show the results of the life cycle cost (LCC) of the unit for the cities of Heraklion, Athens, Thessaloniki, and Kastoria, respectively. The general image of these figures indicates that the increase of the area leads generally to lower LCC and in some  
Figure 17: Auxiliary energy consumption with optimum tilt angles for advanced FPC. Figure 18: Auxiliary energy consumption with optimum tilt angles for ETC. cases, it is minimized for collecting areas in the range of 30 to 40 m\({}^{2}\). Moreover, the most important conclusion from the LCC analysis is that the advanced FPC is the best choice for all the locations. This result is a very interesting one because it indicates the selection of the advanced FPC for all the possible examined cases. This is an efficient collector with an acceptable cost and thus the economic evaluation makes it the most appropriate choice. Figure 19: Life cycle cost for different collecting areas and collector types for Heraklion. Figure 20: Life cycle cost for different collecting areas and collector types for Athens. Table 7 includes the results for the optimal operation according to the LCC criterion and more specifically, the best design for every collector type is given. For the Heraklion, the minimum LCC is 11,088 \(\upxi\) for the 35 m\({}^{2}\) of advanced FPC, with the 40 m\({}^{2}\) of simple FPC to follow at 12,383 \(\upxi\) and the less viable choice is the 30 m\({}^{2}\) of ETC with 13,480 \(\upxi\). For Athens, the minimum LCC is 12,875 \(\upxi\) for advanced FPC and 40 m\({}^{2}\), with the 40 m\({}^{2}\) of simple FPC to follow at 14,737 \(\upxi\) and the last choice is the 35 m\({}^{2}\) of ETC with 15,518 \(\upxi\). For Thessaloniki, the minimum LCC is found for advanced FPC at 16855 \(\upxi\) for 40 m\({}^{2}\), with ETC to follow at 19,329 \(\upxi\) at 40 m\({}^{2}\) and simple FPC at 19,405 \(\upxi\) at 40 m\({}^{2}\). For Kastoria, the advanced FPC leads to the minimum LCC at 17,692 \(\upxi\) with 40 m\({}^{2}\), with 40 m\({}^{2}\) ETC to follow at 20,110 \(\upxi\) and 40 m\({}^{2}\) simple FPC to follow at 20,784 \(\upxi\). It is obvious that the advanced FPC is the best solar technology for all the locations. For this reason, Figure 23 shows the LCC for the four locations with this collector. It is concluded that the lower LCC is found for Heraklion, with Athens, Thessaloniki, and Kastoria to follow, respectively. Figure 21: Life cycle cost for different collecting areas and collector types for Thessaloniki. Figure 22: Life cycle cost for different collecting areas and collector types for Kastoria. Figure 24-27 depict the results of the simple payback period (SPP) of the unit for the cities of Heraklion, Athens, Thessaloniki, and Kastoria, respectively. Moreover, Table 7 gives the results for the SPP for the cases with minimum LCC. The SPP, for all the locations and collector types, has an increasing rate with the increase of the collecting area. Moreover, the minimum SPP is found for the simple FPC. These results make clear that the overall minimum SPP is found for the cheapest choice with a 5 m\({}^{2}\) simple FPC. However, these designs are not acceptable according to energy analysis because the solar coverage is low in these cases. Therefore, a multi-objective optimization study is presented in the next section.