## 3 Results and Discussion
### Parametric Analysis for the Climate Zone B--Athens  
The first part of the parametric analysis regards the presentation of the basic impact of the collecting area and of the tilt angle on the results. The location of Athens (climate zone B) is selected to be used as the example city in order to present this initial parametric analysis. Figure 4 includes the results about the influence of the solar field area (A\({}_{\text{col}}\)) on the yearly solar coverage (F) for different tilt angles (\(\beta\)) for Athens and for the simple FPC. Figure 5 is the respective depiction for the advanced FPC, while Figure 6 is the respective depictionfor the ETC. These three figures lead to similar quality conclusions, which indicate that a higher collecting area increases the solar coverage, while the impact of the tilt angle is not so important to the results. However, among the examined cases, different optimum tilt angles can be found, something that indicates this parameter is an important one. Figure 4 shows that for low collecting areas, the optimum tilt angle is at 30\({}^{\circ}\), while for higher collecting areas the optimum value increases, and for the collecting area of 40 m\({}^{2}\), it is at 45\({}^{\circ}\). This is an interesting result that is based on the impact of the tilt angle on the seasonal available solar energy on the collector aperture. More specifically, in low collecting areas, the small tilt angles boost a lot during the summer period when there is great solar potential. However, in high collecting areas, the summer load is covered with all the tilt angles, thus the optimum tilt angle is the one that maximizes the winter performance. In other words, the conclusion that is obtained is that when there is a great collecting area, the angle has to be the best one for the winter period. Similar conclusions are obtained in Figures 5 and 6. More specifically, the advanced FPC and the ETC are more efficient collectors compared to the simple FPC, something that makes possible the optimum tilt angle to be at 50\({}^{\circ}\) during the winter. At this point, it is useful to take into account that the higher tilt angle aids the winter operation because the sun has a lower height during the winter, and so the high tilt angle makes the solar rays more vertical during the winter. Also, please take into account that according to Ref. [27], the optimum winter angle for Athens can be 15\({}^{\circ}\) plus the geographical latitude, so to be 53\({}^{\circ}\). Therefore, the values of 45-50\({}^{\circ}\) are acceptable. Figure 7 summarizes the optimum tilt angles for different collecting areas and collector types for the location of Athens. Generally, the optimum tilt angles are similar for all the collector types. However, it can be said that in higher collecting areas, the more efficient collectors have to be placed with a bit greater tilt angle. Moreover, another conclusion is that the higher collecting leads to a higher optimum tilt angle in order to boost the winter because in the summer there is high coverage for all the examined tilt angles due to the plenty of areas. Figure 4: The impact of the solar field area and of the tilt angle on the yearly solar coverage for Athens and the simple FPC. Figure 5: The impact of the solar field area and of the tilt angle on the yearly solar coverage for Athens and the advanced FFC. Figure 6: The impact of the solar field area and of the tilt angle on the yearly solar coverage for Athens and the ETC. Figure 8 shows the yearly solar coverage for the optimized cases for the location of Athens, while Figure 9 depicts the respective results for the auxiliary energy consumption by the electrical heater. Figures 8 and 9 make clear that a higher collecting area makes the solar coverage higher, while the auxiliary consumption is lower. The increase of the solar coverage has a decreasing rate because, after the limit of 15 m\({}^{2}\), it is not possible to utilize all the useful heat from the extra collecting area.