# Investigation of Different Storage Systems for Solar-Driven Organic Rankine Cycle
## 3 Results and Discussion
### Final Comparison of the Three Examined Storage Systems
Therefore, there was a need to conduct a more detailed analysis with a multi-objective depiction of energy and financial criteria together. The system efficiency and the NPV were selected to be used in a two-dimensional depiction, which is given in Figure 15. The goal was to determine the choices that maximized both these criteria. So, a Pareto front was created with five design points: three points with thermal oil-ceramic rock and two points with PCM. Table 8 summarizes the optimum points of the Pareto front and also includes the symbols of these cases that correspond to the depiction of Figure 15. It can be said that every point in the Pareto front had a better index than the others; so there was not any point among them that had the same two indexes as the other point. So, all these five designs were optimum and the selection of one of them was based on extra criteria, such as the availability and the long-term reliability of every technology. It can be said that the use of ceramic rocks presented more optimum points in the Pareto front, and also it was the best design according to the single-criterion, and therefore it seemed to have precedence in the final selection. Generally, the results were better for the cases with 8 m\({}^{3}\), and thus these results are explained in detail below. For the design with the 160 m\({}^{2}\) collecting area and 8 m\({}^{3}\) storage tank volume, the thermal oil-ceramic rocks design led to 14.79% system efficiency and net present value to 123.52 k\(\epsilon\), the phase change material storage to 14.65% and 119.16 k\(\epsilon\), respectively, while the pure thermal oil case led to 13.57% and 104.37 k\(\epsilon\), respectively. So, in this case, the electricity production enhancement with ceramic rocks compared to the pure thermal oil was 8.99%, while the enhancement with PCM compared to the pure oil was 7.95%. For the design with a 180 m\({}^{2}\) collecting area and 8 m\({}^{3}\) storage tank volume, the thermal oil-ceramic rocks design led to 13.89% system efficiency and net present value to 129.73 k\(\epsilon\), the phase change material storage to 13.97% and 128.66 k\(\epsilon\), respectively, while the pure thermal oil case led to 12.48% and 105.32 k\(\epsilon\), respectively. So, in this case, the electricity production enhancement with ceramic rocks compared to the pure thermal oil was 11.3%, while the enhancement with PCM compared to the pure oil was 11.9%. For the design with a 200 m\({}^{2}\) collecting area and 8 m\({}^{3}\) storage tank volume, the thermal oil-ceramic rocks design led to 12.97% system efficiency and net present value to 132.36 k\(\epsilon\), the phase change material storage to 13.07% and 131.83 k\(\epsilon\), respectively, while the pure thermal oil case led to 11.54%  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline _V (m\({}^{3}\))_ & _A\({}_{c}\) (m\({}^{2}\))_ & \(\eta_{sys}\) & _NPV (k\(\epsilon\))_ & _LCOE (EUR/kWh)_ & _PP (Years)_ \\ \hline
8 & 160 & 14.79\% & 123.52 & 0.0891 & 7.56 \\
8 & 180 & 13.89\% & 129.73 & 0.0900 & 7.63 \\
8 & 200 & 12.97\% & 132.36 & 0.0919 & 7.84 \\
10 & 160 & 14.63\% & 119.17 & 0.0921 & 7.87 \\
10 & 180 & 13.79\% & 126.10 & 0.0923 & 7.89 \\
10 & 200 & 12.95\% & 130.05 & 0.0938 & 8.04 \\ \hline \hline \endand 118.91, kE respectively. In this case, the electricity production enhancement with ceramic rocks compared to the pure thermal oil was 12.4%, while the enhancement with PCM compared to the pure oil was 13.3%. Furthermore, it is important to note that when the collecting area was varied from 160 m\({}^{2}\) to 200 m\({}^{2}\) with the tank volume at 8 m\({}^{3}\), the efficiency enhancement with ceramic rocks compared to pure oil ranged from 8.99% up to 12.39%, while the enhancement with PCM ranged from 7.96% to 13.26%. For the same conditions, the NPV was improved with ceramic rocks from 18.35% to 25.79%, while with PCM from 14.17% to 25.29%.