## 3 Experimental Results  
The solid particles used in this study are called Carbobead CP (also known as AC-UCCAST ID); they are engineered ceramic particles manufactured by CARBO Industrial Technologies. The chemical composition of Carbobead CP particles is listed in Table 3.  
Several tests were conducted to identify the relevant kinematic, optical, and thermophysical properties of the Carbobead CP particles. For instance, the angle of repose was measured using different methods including the hollow cylinder, fixed funnel, and revolving cylinder methods. The packed-bed reflectivity was measured using ASD FieldSpe3 Portable Spectroradiometer. The absorptance was estimated using the measured reflectivity. Some of the measured properties, and those provided by the manufacturer, are listed in Table 4. Moreover, the particles did not show any sign of agglomeration while being tested in the PHR and inside an electrical oven at high temperatures. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{\begin{tabular}{c} **Static Angle** \\ **of Repose** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Dynamic Angle** \\ **of Repose at** \\ **48 RPM** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Internal** \\ **Angle of** \\ **Repose** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Packed-Bed** \\ **Absortance** \\ **\%** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Tapped Bulk** \\ **Density** \\ **[g/cm\({}^{3}\)]** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Material** \\ **Density** \\ **[g/cm\({}^{3}\)]** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Sphericity** \\ **(mm)** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Size** \\ **(mm)** \\ \end{tabular} } \\ \hline
29 & 57.4 & 30.5 & 85.25 & 1.88 & 3.27 & 0.9 & 0.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Carbobead CP properties. Figure 4: Temperature variation during the startup/preheating process. A series of on-sun and steady-state tests were performed to investigate the thermal performance of the heat exchanger. The recuperated air of the T100 unit was used as the heat source in the steady-state tests while the solar energy was the heat source in the on-sun (unsteady) tests. Several mass flow rates of particles were used to investigate the effect of flow rate (velocity) variation on the thermal performance of the heat exchanger. As mentioned earlier, the particle flow rate can be controlled by controlling the rotational speed of the Olds Elevator. The Olds Elevator has a variable frequency drive that controls the motor speed of the Olds Elevator; each speed/frequency gives a certain particle flow rate. Figure 5 shows how the particle flow rate changes with the speed/frequency of the elevator; five flow rates were measured while the particles were maintained at around 500 \({}^{\circ}\)C; it was found that the flow rate changes linearly with the speed. Another four flow rates have been extrapolated using such linear relationship as can be seen in Figure 5. The recuperated air of the T100 unit always enters the heat exchanger with constant conditions, i.e., constant flow rate, constant temperature, and constant pressure. The air and particle temperatures, air pressure, and air flow rate are continuously measured and recorded during the tests. The quality of the thermal insulation of the heat exchanger was tested by keeping the particles inside the heat exchanger without being circulated. Meanwhile the T100 unit was kept running in order for the heat exchanger to reach a thermal equilibrium. Once this was conducted, the T100 unit was shut down, and air and particle temperatures were continuously recorded for several hours in order to examine the temperature decay process. Figure 6 shows the results of this test; the air temperature \(T_{a}\) is the arithmetic mean of the readings of the thermocouples installed below and above the middle of the heat exchanger, i.e., \(T_{a,2}\) and \(T_{a,3}\), respectively.