## 3 Results and Discussion
### Comparison of the Energy Performance for Different Climate Zones  
After the initial parametric analysis for Athens (Zone B), the results of the four climate zones are presented in Section 3.2. It is notable to state that the yearly demand for useful heat for the building is 15,295 kWh for Heraklion, 17,301 for Athens, 18,687 kWh for Thessaloniki, and 20,037 for Kastoria. These values are depended on the water temperature from the grid because the colder climate zones have lower water temperatures, and so higher energy amounts are needed for warming the water up to the desired temperature level. Figures 10-12 show the yearly solar coverage for the simple FPC, the advanced FPC, and the ETC, respectively. These results concern the operation with the optimum tilt angle in every case which maximizes the yearly solar coverage. According to the results for Figures 10-12, the solar coverage is higher for Heraklion with Athens, Thessaloniki, and Kastoria to follow, respectively. These results are reasonable because the solar coverage is higher in warmer locations with higher potential and higher temperatures. It is critical to state that the difference between Thessaloniki and Kastoria is very small for all the collector types. At this point, it is important to state that for Heraklion, the maximum solar coverage  
Figure 7: Optimum tilt angles aiming the solar coverage maximization for Athens. is found at 96.75%, for Athens at 94.45%, for Thessaloniki at 87.98%, and for Kastoria at 87.41%; all these maximum values for 40 m\({}^{2}\) collecting area and ETC. Figure 8: Yearly solar coverage with optimum tilt angles for the location of Athens. Figure 9: Auxiliary energy consumption with optimum tilt angles for the location of Athens. Figure 11: Yearly solar coverage with advanced FPC for the examined locations with optimized tilt angle. Figure 10: Yearly solar coverage with simple FPC for the examined locations with optimized tilt angle. The next presented results regard the values of the optimum angles for all the locations and the collector types. Figure 13 shows the optimum angles with the simple FPC, Figure 14 with the advanced FPC, and Figure 15 with ETC. The general result is that a greater collecting area leads to a higher optimum tilt angle. Moreover, for the same collector type and collecting area, the optimum tilt angle is similar for the various locations, with small deviations of \(5^{\circ}\) in some cases. The last step concerns the results of the auxiliary energy consumption on the yearly basis for the different cases with the optimized tilt angle. It is important to state that the maximization of the yearly solar coverage leads to the minimization of the yearly auxiliary energy consumption by the electrical heater. Figures 16-18 show the results of the auxiliary energy consumption of the simple FPC, the advanced FPC, and the ETC, respectively. For all the collector types, the lowest auxiliary energy consumption is found for Heraklion, with Athens, Thessaloniki, and Kastoria to follow. For the case with ETC and 40 m\({}^{2}\), the lowest consumptions are found at 497, 961, 2245, and 2522 kWh for Heraklion, Athens, Thessaloniki, and Kastoria, respectively. Kastoria has the greatest consumption and there is a notable difference with Thessaloniki, while the respective difference in the solar coverage was very small. This result can be explained by the different needs among the locations, so it is obvious that the results about solar coverage and auxiliary energy consumption have the same trends but there are small deviations regarding the distance between the curves. In any case, the final conclusions indicate that the auxiliary energy consumption is lower in the warmer climate zones. Figure 12: Yearly solar coverage with ETC for the examined locations with optimized tilt angle. Figure 14: Optimum tilt angles for maximizing the solar coverage with advanced FPC for the examined locations. Figure 13: Optimum tilt angles for maximizing the solar coverage with simple FPC for the examined locations.