# Concentrating Solar Collectors for a Trigeneration System--A Comparative Study
## 3 Results and Discussion
### Monthly and Yearly Performance of the Studied Cases  
The next step in this work is the presentation of the results on a monthly basis. Figure 8 shows the collector's thermal efficiency for all the months. The LFR had an extremely low performance during the winter period due to the relatively low solar altitude. More specifically, in December the LFR had 9.14% thermal efficiency and in June, 49.96%. The solar dish had an approximately constant performance throughout the year which was close to 50%. The PTC presented 29.19% thermal efficiency in December and 60.11% in June. It is remarkable to state that the PTC was better than the dish for seven months from March up to September. At this point, it must be stated that the optical efficiency variation due to the different sun angles during the year had the highest influence on collector efficiency. The solar dish operated with a two-axis tracking system and thus it had maximum optical efficiency throughout the year. On the other hand, the PTC and LFR had single-axis tracking systems because they were linear concentrating systems and thus they had optical losses due to the different sun altitudes. Moreover, the LFR had higher optical losses due to the tracking restrictions of the mirrors which were close to the ground. So, the variation of the collector efficiency was higher in the LFR throughout the year, it was lower in the PTC and it was extremely low in the dish. Furthermore, it must be stated that the ambient temperature and the solar irradiation level were variable during the year but their impact on the collector efficiency was low due to the low values of the thermal losses in concentrating systems. On the other hand, the solar irradiation potential had a high influence on the useful heat production because the solar irradiation potential was multiplied with the mean monthly efficiency value in order to give the monthly yield in useful heat production. The system's electricity production is given in Figure 9 for all the collector types. This figure has a different shape than Figure 8 because the solar energy amount of every month was taken into consideration in every case. The winter period is a period with a low number of sunny days and so the electricity production is too low due to the low available solar energy and the low thermal efficiency. The maximum electricity production was found in July at 3238 kWh with the PTC, while 2677 kWh with the DISH (solar dish) and 2078 kWh with LFR. On the other hand, the electricity production in December was 491 kWh, 929 kWh and 69 kWh with the PTC, DISH and LFR, respectively. Figure 8: Monthly collector thermal efficiency for the three solar collectors. Figure 10 depicts the heating production of the examined cases for all the months. The results of Figure 10 are proportional to the electricity results that are given in Figure 9. The maximum heating production was found in July at 8447 kWh with the PTC, while 6982 kWh with the DISH and 5419 kWh with the LFR. On the other hand, the heating production in December was 1282 kWh, 2422 kWh and 180 kWh with the PTC, DISH and LFR, respectively. Figure 11 illustrates the cooling production of the examined cases for all the months. The results of Figure 11 are proportional to the results of Figures 9 and 10 of the electricity and the heating production, respectively. The maximum cooling production was found in July at 7814 kWh with the PTC, while 6459 kWh with the DISH and 5013 kWh with the LFR. On the other hand, the cooling production in December was 1186 kWh, 2241 kWh and 166 kWh with the PTC, DISH and LFR, respectively. Figure 10: Monthly heating production for the three solar collectors. Figure 9: Monthly electricity production for the three solar collectors. Figure 12 exhibits the system energy efficiency during the months. This parameter follows similar trends with the collector thermal efficiency, as in Figure 8. The maximum energy efficiency was found in June at 76.61% with the PTC, while 49.88% with the LFR and 62.96% with the DISH.