## 4 Conclusions  
A 50 kWh moving packed-bed particle-to-air heat exchanger, fabricated by Solex Thermal Science Inc., has been integrated into the PBCSP test facility located at KSU in Riyadh, Saudi Arabia. The heat exchanger incorporates a shell-and-tube configuration in which solid particles move downward inside vertical tubes while air flows on a shell-side. Particles, called Carbobead CP, with 0.3 mm mean diameter were used in this investigation. The thermal performance of the heat exchanger was evaluated using the recuperated air of the T100 unit as the heat source. The effect of particle flow rate variation on the overall heat transfer coefficient and effectiveness was studied. Based on LMTD calculations, the measured overall heat transfer coefficient, in the steady-state tests, was found to be in a range that meets the value targeted by the DOE (100 W/m\({}^{2}\)-\({}^{\circ}\)C). The accuracy of measurement was investigated by repeating some tests; a slight difference was noted in the measured U. The results revealed that a significant portion of the heat exchange took place at the bottom section of the heat exchanger. In addition, a small pressure drop (5 kPa) was recorded on the air side of the heat exchanger. Achieving a steady-state condition during on-sun operation currently is not possible. In the future, the amount of solar energy is to be controlled. By controlling the number of operating heliostats, the PHR's outlet, thus, the exchanger's inlet temperature, can be held constant during the test. Going with such measures allows for steady-state operation to be reached, i.e., the performance of the exchanger can be evaluated at elevated temperature (>650 \({}^{\circ}\)C). Conceptualization, S.A., H.A.-, S.J., Z.A.-S., A.B., N.J. and D.M. ; data curation, N.S.S. ; formal analysis, N.S.S. and S.A.; funding acquisition, H.A.-, investigation, N.S.S., S.A. and E.D. ; methodology, N.S.S., S.A., H.A.-, Z.A.-S., O.Z. and S.J. ; project administration, S.A. and H.A.-, resources, H.A.-, supervision, Z.A.-S., O.Z. and H.A.-, validation, N.S.S., S.A., Z.A.-S. and O.Z. ; visualization, N.S.S. ; writing--original draft, N.S.S. ; writing--review & editing, N.S.S., S.A., E.D., R.S.S., Z.A.-S., O.Z., A.A., E.-L, S.D., A.B. and N.J. All authors have read and agreed to the published version of the manuscript. Figure 12: Repeatability analysis of steady-state tests. Funding: This research was funded by \"The Deanship of Scientific Research at King Saud University, through research group no. RG-1440-087\", which is truly appreciated. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding this work through research group no. RG-1440-087. Conflicts of Interest: The authors declare no conflict of interest. References  
* [PERSON] et al. 2016 [PERSON]; [PERSON] [PERSON]; [PERSON]; [PERSON] Review of heat transfer fluids in tube-receivers used in concentrating solar thermal systems: Properties and heat transfer coefficients. _Renew. Sustain. Energy Rev._**2016**, _55_, 298-315. [CrossRef]
* [PERSON] et al. 2019 [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON]; [PERSON]; [PERSON] [PERSON]; [PERSON]; [PERSON] [PERSON]; [PERSON] Thermal performance evaluation of lining materials used in thermal energy storage for a falling particle receiver based CSP system. _Sol. Energy_**2019**, _178_, 268-277. [CrossRef]
* 3 \"Generation 3 Concentrating Solar Power Systems (Gen3 CSP) Phase 3 Project Selection\". Energy.gov, Solar Energy Technologies Office.