## 3 Results and Discussion
### Daily, Monthly and Yearly Analysis  
The investigated unit is studied for the default design scenario on a daily, monthly and yearly basis for the city of Athens (Greece). The simulation was performed by using 12 typical days, one for every month and by assuming that the unit operates only on the sunny days of every month according to the weather data of the studied location. Figure 17 illustrates the daily performance of the unit for the mean day of September which is generally a representative day for all the year as it has been observed by the present analysis. It can be said that solar irradiation presents maximum value at the solar noon while the electricity production has a relatively stable profile from 8:00 up to 16:00. This result can be explained by the impact of the incident angle modifier on the results; this parameter has minimum values close to the solar noon and higher earlier in the morning and later in the afternoon. Practically, the single-axis tracking system with the collector main axis in the south-north direction eliminates the optical losses in the morning and in the afternoon but close at solar noon it is not able to reach very high values due to the sun altitude. Therefore, the optical efficiency is reduced at solar noon but the higher solar irradiation level at this period makes generally electricity production have a relatively stable profile, something important for the usual applications. The system efficiency is influenced by both the electricity and the solar rates and the results show maximization in the morning and the afternoon. The mean system efficiency is found at 20.19%, the maximum at 22.37% and at the solar noon at 18.37%. These deviations are not high, but they are remarkable. The next step is the monthly and yearly investigation of the system. Figure 18 shows the monthly electricity production and the monthly system efficiency, while Table 8 includes the respective results. Moreover, Table 8 includes the results for the mean day and for all the months. At this point, it has to take into account that the number of sunny days every month is used for the calculation of the monthly results. This fact makes the calculation more complex and proves that all the mean monthly days have not the same contribution to the yearly results. The maximum monthly production is found for July with 13,224 kWh,  
Figure 17: Daily variation of the solar energy input, electricity production and system efficiency for the mean day of September at the default design scenario. the second month is August with 12,373 kWh, while June follows with 10,202 kWh. The lowest yield is found for January with 2491 kWh while December follows with 2549 kWh and February is close with 2630 kWh. On the other hand, the system efficiency is maximized for June at 22.75% with July and May to follow with close values at 22.69% and 22.63% respectively. August has a reduced efficiency at 22.1% and the lowest efficiency is found for December at 11.95%. Generally, the efficiency and the electricity yield are maximized in the summer, and they are minimized in the winter; reasonable and acceptable results are in accordance with the solar potential availability in Athens. The yearly efficiency is found to be 19.80% and the yearly electricity yield for the 226 sunny days in Athens is 81,164 kWh.