## 4 Conclusions  
There are many benefits to using solid particulates as an HTM in CSP systems, making them a better choice than traditional heat transfer media like steam, oil, air, and molten salt. These advantages encompass the capacity to operate at exceptionally high temperatures (>1000 \({}^{\circ}\)C), widespread availability, cost effectiveness, high thermal energy storage capabilities, absence of freezing constraints, and direct compatibility with solar irradiation. These attributes collectively contribute to significantly enhanced operational efficiencies and reduced levelized cost of energy expected from particle-based CSP plants. On the other hand, a crucial requirement is the thorough assessment of the characteristics of possible solid particulates meant for application in such severe-heat environments. In this article, four naturally occurring particulate materials--Riyadh white sand, Riyadh red sand, Saudi olivine sand, and US olivine sand--and one synthetic particulate material called Carbbead were investigated. All natural particulate materials were sifted to the targeted particle size for the 1.3 MWe pre-commercial plant that will be built in Waad-Al-Shamal, Saudi Arabia [36]. In particular, the relevant ambient- and high-temperature thermophysical properties were thoroughly examined. The parameters that were examined include thermal conductivity, sintering temperature, actual density, tapped bulk density, and loose bulk density. This meticulous analysis aimed to ascertain the suitability of these materials for deployment in high-temperature CSP applications, thereby advancing the understanding and subsequent design and optimization of particle-based CSP and similar heat transfer systems. The results revealed or confirmed the following:  
1. [leftmargin=*]
2. Particulate size and size distribution have a significant effect on bulk density. For most of the particles, their porosity ranges from 35% to 45%, consistent with the commonly expected value of 40% for monodisperse particles. In contrast, the porosity of the USOS material is much lower, 24% loose and 18% tapped, which is indicative of the presence of fines in the mix filling the spaces that are voids between more uniform particles. 2. The tapped bulk density for all particle materials is always greater than the loose density for all particles. In the case of 0.5 mm CB, the beads are so uniform and smooth that the effect of tapping is minimal. 3. During the sintering test, when the temperature and pressure were raised to 1300 \({}^{\circ}\)C and 40 MPa, respectively, for RWS (0.5-1 mm), the specimen did not exhibit any signs of sintering. For RWS (0.21-0.425 mm), only weak binding formed at the high pressure of 50 MPa. Sintered RRS was produced at 40 MPa and 1300 \({}^{\circ}\)C, and CB at 20 MPa and 1100 \({}^{\circ}\)C. Similar behavior was displayed by SOS and RRS. At 800 \({}^{\circ}\)C, USOS solidified, while at higher temperatures, it melted. This demonstrates why USOS cannot be used in high-temperature particle-based CSP systems as TES or an HTM. 4. Particulate materials' thermal diffusivity and thermal conductivity decrease as the temperature rises, reaching their lowest values at the highest temperatures. This indicates that a bigger-size heat exchanger is needed to run at elevated temperatures. When comparing various particulates, RWS has the lowest thermal conductivity and minimum thermal diffusivity of 0.15 mm\({}^{2}\)/s and 0.46 W/mK, respectively, at the maximum temperature of 475.5 \({}^{\circ}\)C. Conversely, at the same temperature, the maximum values of the RRS are 1.25 W/mK and 0.432 mm\({}^{2}\)/s, respectively. For future studies, it is planned to upgrade the transient laser flash apparatus at KSU for the measurement of the thermal diffusivity of loose particles.