## 3 Results
It should be noted that the result is therefore based on the specific mass flow rate which was investigated in this work. Furthermore, the application of SiO2/oil nanofluid had the smallest effect on improving the ORC performance, while for the cubical cavity receiver, using Al2O3/oil, had the largest effect on improving the ORC performance. The cubical cavity receiver, using Al2O3/oil is, therefore, recommended as the heat source for the investigated solar ORC with the specific solar heat transfer fluid mass flow rate which was investigated in this work. The calculated results related to enhancement of the solar system performance can be compared with reported results by Ref. [42]. [PERSON] and [PERSON] [42] investigated performance of a solar concentrator system using different nanofluids including 3% Al2O3/Oil, 3% TiO2/Oil, and 1.5% Al2O3/Oil and 1.5% TiO2/Oil. They reported improvement lower than 1% for the investigated solar system using different nanofluids. The cavity receivers have very small thermal losses and so there is not such a high thermal enhancement margin. Therefore, the use of nanofluids as a thermal enhancement method can enhance the performance up to 2%-3% maximum. The calculated results can be compared with the results reported in Ref. [43], where the effect of alumina/oil nanofluid, with different size and volume fractions, was investigated as solar working fluid for a solar ORC. In the current research, performance of the solar ORC system using different nanofluids including oil/Al2O3, oil/CuO, and oil/SiO2 nanofluid of the solar working fluid is a new subject for assessment. Finally, Figure 14 shows the variation of overall efficiency improvement of the solar ORC system with the variation of nanofluid concentration. The cubical cavity receiver was used as the dish absorber. Different nanofluids including oil/Al2O3, oil/CuO, and oil/SiO2 were considered as the solar working fluid. The solar system was investigated at working fluid inlet temperature of 40 \({}^{\circ}\)C, and solar radiation of 632.97 W/m\({}^{2}\). The ORC system was considered at constant turbine inlet temperature of 229 \({}^{\circ}\)C, \begin{table}
\begin{tabular}{c c c c c} \hline  & **Oil/Al2O3** & **Oil/CuO** & **Oil/SiO2** \\ \hline Hemispherical & 0.49\% & 0.49\% & 0.36\% \\ Cubical & 1.92\% & 1.63\% & 1.15\% \\ Cylindrical & 1.66\% & 2.09\% & 1.35\% \\ \hline \end{tabular}
\end{table} 
Table 14: Percentage improvement of the ORC net power output for the different shapes of the cavity receivers with volume fraction of 3% for weather conditions of 20 October 2016, Tehran, Iran. \begin{table}
\begin{tabular}{c c c c} \hline  & **Oil/Al2O3** & **Oil/CuO** & **Oil/SiO2** \\ \hline Hemispherical & 0.49\% & 0.49\% & 0.36\% \\ Cubical & 1.92\% & 1.63\% & 1.15\% \\ Cylindrical & 1.66\% & 2.09\% & 1.35\% \\ \hline \end{tabular}
\end{table} 
Table 15: Percentage improvement of the overall efficiency of the different cavity receivers with volume fraction of 3% for weather conditions of 20 October 2016, Tehran, Iran. \begin{table}
\begin{tabular}{c c c c} \hline  & **Oil/Al2O3** & **Oil/CuO** & **Oil/SiO2** \\ \hline Hemispherical & 0.49\% & 0.49\% & 0.36\% \\ Cubical & 1.92\% & 1.63\% & 1.15\% \\ Cylindrical & 1.66\% & 2.09\% & 1.35\% \\ \hline \end{tabular}
\end{table} 
Table 13: Variation of the thermal parameters and pressure drop for cylindrical receiver using nanofluids with volume fraction of 3% for weather conditions of 20 October 2016, Tehran, Iran. and turbine inlet pressure of 2.5 MPa. Methanol was used as the ORC working fluid. As shown in Figure 14, the thermal efficiency resulted higher improvement using application of Al\({}_{2}\)O\({}_{3}\)/oil nanofluid with higher nanofluid concentration. Generally, thermal performance improvement was calculated to be between 1% and 2%, as was reported in results by Ref. [42].