## 3 Results
The hemispherical, cylindrical, and cubical cavity receivers were used as the ORC heat source. Different nanofluids including oil/Al2O3, oil/CuO, and oil/SiO2 nanofluids were considered as the solar working fluid with nanofluid concentration of 3% volume fraction. The solar system was investigated at solar radiation of 632.97 W/m2, working fluid inlet temperature of 40 \({}^{\circ}\)C, and working fluid flow rate of 60 mL/s. The ORC system was considered at constant turbine inlet temperature of 229 \({}^{\circ}\)C, and turbine inlet pressure (TIP) of 2.5 MPa. Table 11 displays the variation of the thermal parameters of the hemispherical cavity receiver using different nanofluids (oil/Al2O3, oil/CuO, and oil/SiO2) (also see Tables 12 and 13 for the cubical and cylindrical receiver, respectively). Note that the thermal performance, in terms of cavity heat gain, thermal efficiency, and outlet temperature of the solar working fluid, has been increased slightly by the application of nanofluids. Also, note that the pressure drop of the solar system is increased by the application of nanofluids, when compared to pure oil. As seen, the cubical cavity receiver has the highest thermal performance using oil/Al2O3 nanofluid as the solar working fluid. Similar studies were conducted (see Refs. [40; 41]) where the influence of nanofluid application, as solar working fluid of a dish concentrator with a spiral cavity receiver, was investigated using energy and exergy analyses. Similar results were reported by Refs. [40; 41]. In the current study, application of different oil-based nanofluids as heat source of an ORC system with different shapes of cavity receivers as the ORC heat source is presented as a new subject for study. \begin{table}
\begin{tabular}{c c c c} \hline  & **Hemispherical** & **Cubical** & **Cylindrical** \\ \hline \(\dot{Q}_{net}\) (W) & 1095 & 1118 & 1044 \\ \(\eta_{th,rec}\) & 0.68 & 0.69 & 0.65 \\ \(T_{out,rec}\) (\({}^{\circ}\)C) & 50.77 & 51.00 & 50.42 \\ \(T_{inlet,rec}\) (\({}^{\circ}\)C) & 40 & 40 & 40 \\ \(\dot{m}\) (mL/s) & 60 & 60 & 60 \\ \(\Delta P\) (Pa) & 1562 & 10633 & 7547 \\ \(T_{s,ave}\) (\({}^{\circ}\)C) & 286.70 & 106.41 & 116.08 \\ \hline \end{tabular}
\end{table} 
Table 10: The thermal parameters and pressure drop for the cavity receivers with pure thermal oil using the weather conditions of 20 October 2016, Tehran, Iran. \begin{table}
\begin{tabular}{c c c c c} \hline  & **Oil** & **Oil/Al2O3** & **Oil/CuO** & **Oil/SiO2** \\ \hline \(\dot{Q}_{net}\) (W) & 1094.80 & 1100.11 & 1100.21 & 1098.75 \\ \(\eta_{th,rec}\) & 0.680 & 0.683 & 0.683 & 0.683 \\ \(T_{out,rec}\) (\({}^{\circ}\)C) & 50.77 & 50.01 & 49.25 & 50.50 \\ \(\Delta P\) (Pa) & 1561.62 & 1717.93 & 1864.63 & 1637.66 \\ \hline \end{tabular}
\end{table} 
Table 11: Variation of the thermal parameters and pressure drop for hemispherical receiver using nanofluids with volume fraction of 3% for weather conditions of 20 October 2016, Tehran, Iran. \begin{table}
\begin{tabular}{c c c c c} \hline  & **Oil** & **Oil/Al2O3** & **Oil/CuO** & **Oil/SiO2** \\ \hline \(\dot{Q}_{net}\) (W) & 1117.69 & 1139.16 & 1135.96 & 1130.57 \\ \(\eta_{th,rec}\) & 0.694 & 0.708 & 0.706 & 0.702 \\ \(T_{out,rec}\) (\({}^{\circ}\)C) & 51.00 & 44.72 & 49.56 & 50.81 \\ \(\Delta P\) (Pa) & 10632.68 & 25277.24 & 12695.80 & 11150.45 \\ \hline \end{tabular}
\end{table} 
Table 12: Variation of the thermal parameters and pressure drop for cubical receiver using nanofluids with volume fraction of 3% for weather conditions of 20 October 2016, Tehran, Iran. A comparison of the percentage improvement of the ORC net power output, and overall efficiency between three cavity receivers, is presented in Tables 14 and 15, respectively. It is shown that the different types of nanofluids did not have a significant effect on improving the ORC overall efficiency. This is due to the short length of the cavity tube and the short amount of time in which the nanofluid has to absorb the solar thermal energy.