## 2 Materials and Methods
### Validation with Experimental Results
The specifics of the materials used in the construction of the cavity receiver are given in Table 8.  
A comparison between measured experiment data and calculated numerical results are presented in Table 9. It should be mentioned that actual ambient conditions were inputted into the numerical model and used to obtain numerical results. As shown, the mean thermal efficiency deviation is 3.34% which is acceptable for the accuracy model. Also, the deviation variation during the day is presented in Figure 6. As concluded from Table 9, there are acceptable values at noon when the cavity receiver is in the steady-state condition. \begin{table}
\begin{tabular}{c c c} \hline
**Instrument** & **Accuracy** & **Range** & **\% Uncertainty** \\ \hline K-type thermocouples & \(\pm\)0.55 \({}^{\circ}\)C & 0–800 \({}^{\circ}\)C & 0.25 \\ Solar power meter & \(\pm\)0.1 W/m\({}^{2}\) & 0–2000 W/m\({}^{2}\) & 0.25 \\ Anemometer & \(\pm\)0.2 m/s & 0.9 to 35.0 m/s & 10 \\ Volume flow meter & \(\pm\)0.05 mA & 0–20 mA & 1 \\ \hline \end{tabular}
\end{table} 
Table 7: Accuracies and ranges of the measuring instruments [36]. \begin{table}
\begin{tabular}{c c} \hline
**Used Instrument** & **Properties** & **Reason** \\ \hline \multicolumn{3}{c}{Stage 1} \\ \hline Copper tube & Thermal conductivity of 386 W/m.K & High conductivity \\ \cline{2-3}  & High melting point of 1000 \({}^{\circ}\)C & \\ \hline \multicolumn{3}{c}{Stage 2} \\ \hline \multirow{3}{*}{Black chrome coating} & Emittance of 0.1 & \\ \cline{2-3}  & Absorbance of 0.84 & High absorptivity \\ \cline{2-3}  & Stability up to 400 \({}^{\circ}\)C & \\ \hline \multicolumn{3}{c}{Stage 3} \\ \hline \multirow{3}{*}{Mineral wool insulation} & Mineral wool thickness of 0.02 m & \\ \cline{2-3}  & Average insulation conductivity of 0.062 W/m:K & High thermal resistance \\ \hline \end{tabular}
\end{table} 
Table 8: The materials used in the construction of the cavity receiver [36]. \begin{table}
\begin{tabular}{c c} \hline
**Used Instrument** & **Properties** & **Reason** \\ \hline \multicolumn{3}{c}{Stage 1} \\ \hline Copper tube & Thermal conductivity of 386 W/m.K & High conductivity \\ \cline{2-3}  & High melting point of 1000 \({}^{\circ}\)C & \\ \hline \multicolumn{3}{c}{Stage 2} \\ \hline \multirow{3}{*}{Black chrome coating} & Emittance of 0.1 & \\ \cline{2-3}  & Absorbance of 0.84 & High absorptivity \\ \cline{2-3}  & Stability up to 400 \({}^{\circ}\)C & \\ \hline \multicolumn{3}{c}{Stage 3} \\ \hline \multirow{3}{*}{Mineral wool insulation} & Mineral wool thickness of 0.02 m & \\ \cline{2-3}  & Average insulation conductivity of 0.062 W/m:K & High thermal resistance \\ \hline \end{tabular}
\end{table} 
Table 8: The materials used in the construction of the cavity receiver [36]. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline
**Time** & \(T_{ inlet}\) & \(T_{out}\) & \(I_{sum}\) & \(T_{s\;top}\) & \(T_{s\;side}\) & \(T_{amb}\) & \(V_{wind}\) & \(V\) & **ExperimentalNumerical** & **Deviation** \\ \cline{2-3} **(hh:mm)** & (\({}^{\circ}\)C) & (\({}^{\circ}\)C) & (\(W\)/m\({}^{2}\)) & (\({}^{\circ}\)C) & (\({}^{\circ}\)C) & (\({}^{\circ}\)C) & (\({}^{\circ}\)C) & (m/s) & (m/L/s) & \(\eta_{H}\) & \(\eta_{H}\) & \\ \hline
9:30 & 41.10 & 118.10 & 752.82 & 150.00