## 3 Results and Discussion
### Daily and Monthly Operation of the Optimum Scenario  
The performance of the previously determined optimum operation scenario is presented in this section on a daily and monthly basis. For this case, the net electricity load and storage tank volume was equal to 75 kW and 10 m\({}^{3}\), respectively. First of all, three typical days of the year, i.e., 16 February, 15 June and 15 October were selected indicatively to examine both the ORC and net electricity production during the day. The results are illustrated in Figure 17. The difference between these values regards the electricity provided to the PEM water electrolyzer and the hydrogen compressor. Figure 16 depicts that the maximum ORC electricity load during the day was 178.6 kW on 15 June, 104.2 kW on 16 February, and 107.1 kW on 15 October. Additionally, the system operated for only 3.7 h on \begin{table}
\begin{tabular}{c c} \hline \hline
**Outputs** & **Values** \\ \hline Annual net electricity production (\(Y_{el,net}\)) & 210,163 kWh \\ Annual hydrogen energy production (\(Y_{hydrogen}\)) & 78,550 kWh \\ Annual hydrogen production amount (\(M_{hydrogen}\)) & 2356.5 kg \\ Annual energy efficiency (\(\eta_{en,annual}\)) & 14.52\% \\ Annual exergy efficiency (\(\eta_{el,annual}\)) & 15.48\% \\ Capital cost (\(C_{0}\)) & 6633,862 \\ Simple payback period (\(SPBP\)) & 5.80 years \\ Payback period (\(PBP\)) & 6.73 years \\ Internal rate of return (\(IRR\)) & 16.89\% \\ Net present value (\(NPV\)) & \(\xi\)1,073,384 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Outputs of the optimum case. Figure 16: Objective function (\(F\)) values for different values of net electricity load and TES tank volume when the hydrogen price is equal to 25 EUR/kg. 16 February, while on 15 October and 15 June, the operational hours were about 9 and 11.3, respectively; therefore, the ORC electricity production was larger in the summer period compared to the other year's seasons, due to the larger irradiation rates, and the sunshine duration. Moreover, as the net electricity load absorbed by the local grid was constant at 75 kW, the surplus electricity was greater in June, rather than in February or October. Finally, it was obvious that the TES tank contributed to the performance stability. In addition, the hydrogen production rate during the typical days (i.e., 16 February, 15 June and 15 October) is illustrated in Figure 18. According to the results, the maximum hydrogen production mass flow rate during the day was about 1.64 kg/h on 15 June, 0.47 kg/h on 16 February, and 0.51 kg/h on 15 October. It is important to mention that the curves had similar trends to the ORC electricity production curves in Figure 17. As a result, the produced hydrogen amounts that could be stored for later use were higher in June, rather than in February and October. Furthermore, the total net electricity and hydrogen production in kWh, as well as the available solar energy for each month is shown in Figure 19. In general, greater energy values were determined in the summer period for both the products (electricity, and hydrogen) and the inputs (solar irradiation). The maximum values were achieved in July when the net electricity generation was equal to 28,046 kWh, and the hydrogen production in terms of energy was determined at 14,040 kWh. The hydrogen production amount in mass units is presented in Figure 20, and its maximum value, which was equal to 421.2 kg, was defined in July. Figure 20 also depicts the monthly energy and exergy efficiency of the overall configuration. The highest level of energetic and energetic performance was determined in May, with an energy efficiency of 16.4% and an exergy efficiency of 17.46%. At this point, it is remarkable to state that the production profile of the electricity and hydrogen in February was very different compared to the other months. This fact is explained by the restricted solar beam potential during the winter period in Greece.