# Investigation of Different Storage Systems for Solar-Driven Organic Rankine Cycle
## 3 Results and Discussion
### Final Comparison of the Three Examined Storage Systems  
The last part of the results section is devoted to the direct comparison of the three examined storage systems. The previous results in Sections 3.1 and 3.2 were important in order to select the proper organic fluid, the proper rock material, and the proper ORC saturation temperature and to determine the most interesting collecting areas. According to the results of Section 3.2, the collecting area has to be in the range of 160 to 200 m\({}^{2}\) in order to have both high system efficiency and high NPV. Two storage tank volumes were examined and they were 8 m\({}^{3}\) and 10 m\({}^{3}\). It has to be said that this volume was the total volume of the device, which included inside the tubing and all the stored materials. Figure 11 shows the system efficiency for the six examined designs and the three different storage systems (total of 18 scenarios). It has to be said that in all the examined cases, the pure thermal oil case was less efficient than the PCM and the thermal oil-ceramic rock cases. The use of ceramic rocks was found to be the best of the three cases and the PCM for the other three cases, so it is obvious that both these techniques can enhance efficiently the system performance compared to the operation with pure thermal oil storage. The maximum system efficiency was found for the designs with 160 m\({}^{2}\) (with 8 or 10 m\({}^{3}\)) and in these cases, the thermal oil-ceramic rock was the best design. The global maximum system efficiency was found to be 14.79% for the cases [\(A_{c}=160\) m\({}^{2}-V=8\) m\({}^{3}\)], while the case with the PCM was 14.65% and with pure thermal oil was 13.57%. Figures 12-14 present the financial indexes of the examined system for all the scenarios. Figure 12 shows the results of the NPV, Figure 13 the payback period, and Figure 14 the LCOE. Figure 12 indicates that the overall maximum NPV was found for [\(A_{c}=200\) m\({}^{2}-V=8\) m\({}^{3}\)] with thermal oil-ceramic rocks and it was 132.36 k\(\epsilon\). It is important to state that the pure thermal oil led to the minimum NPV in all the cases, and thus it was not a financially attractive choice. Figure 10: Net present value and payback period for different collecting areas with pure thermal oil. Figure 13 indicates that the overall minimum payback period was found for [\(A_{c}\) = 160 m\({}^{2}\) - \(V\) = 8 m\({}^{3}\)] with thermal oil-ceramic rocks, and it was 7.56 years. Moreover, Figure 13 indicates that the overall minimum LCOE was found for [\(A_{c}\) = 160 m\({}^{2}\) - \(V\) = 8 m\({}^{3}\)] with thermal oil-ceramic rocks and it was 0.0891 EUR/kWh. Tables 5-7 include the aforementioned data in order to be clear. Table 5 includes results for the case of pure thermal oil, Table 6 for the case of thermal oil-ceramic rocks, and Table 7 for PCM. Figure 11: System efficiency for different collecting area values and storage tanks with the three examined storage systems. Figure 12: Net present value for different collecting area values and storage tanks with the three examined storage systems. Figure 14: Levelized cost of electricity (LCOE) for different collecting area values and storage tanks with the three examined storage systems. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{\(V\) (m\({}^{3}\))} & \multirow{2}{*}{\(A_{c}\) (m\({}^{2}\))} & \multirow{2}{*}{\(\eta_{sys}\)} & **Net Present Value** & \multicolumn{1}{c}{_LCOE_} & **Payback Period (PP)** \\  & & & **(\(\mathbf{\bm  
The previous analysis proved that only the single-optimization criteria indicated the use of the thermal oil-ceramic rock as the most suitable design, while the use of the pure thermal oil was the less efficient choice. However, it is important to state that the optimum designs (\(A_{c}\), V) were different among the examined criteria, and so there was not a global maximum choice that was the best one with all the criteria.