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Materials 2022, 15, 2946 3 of 20 (SiC), and olivine sand. The results showed significant differences between the various materials, and that the olivine sand had the most favorable characteristics, followed by alumina and cristobalite. Proppants have good optical and mechanical properties to be used as heat transfer medium in concentrated solar power (CSP) systems [24]. They are characterized by a high solar weighted absorptance higher than 90% “as received”, and they are noncorrosive materials. Proppants are available in large quantities; therefore, their cost is relatively low. However, fresh proppants are chemically unstable in oxidiz- ing environments, and their solar weighted absorptance decreases when heated in air at 700 ◦C for many hours. Siegel et al. [24] measured the radiative properties, solar weighted absorptance, and thermal emittance for different manufactured types of proppants, in- cluding CARBOHSP, CARBOACCUCAS, CARBOPROP, and Norton Masterbeads. The measurements of these materials were conducted in two different states, the “as received” state and after heating in the air between 700 ◦C and 1000 ◦C. The results showed a signifi- cant reduction in solar weighted absorptance in the “as received” particles over time and temperature. The effects of aging on the optical and thermal properties were studied for silicon carbide (SiC), silica sand (SiO2), and hematite (Fe2O3) by Palacios et al. [25]. The aging tests were conducted for 500 h at 750 ◦C and 900 ◦C. The results showed that the weighted solar absorptance for black SiC and iron oxide increased and decreased for SiO2 with time. The highest absorptance and specific heat capacity values were recorded for the black SiC after aging at 900 ◦C for 500 h. Nie et al. [26] measured the thermophysical and optical properties of nine different solid-particle samples to be used in a circulating solid-particle receiver. Those were alumina, silicon carbide, quartz sand, desert sand, river sand, yellow ceramist sand, grey ceramist sand, bauxite–cordierite ceramics, and black cop- per slag. Silicon carbide, grey ceramist sand, and bauxite–cordierite ceramics were found to be appropriate materials after the consideration of many factors. The most important factor was the high solar weighted absorptivity, even after heat treatment. Thermophysical, optical, and mechanical properties were measured for several particulate material samples extracted from different locations in the United Arab Emirates by Diago et al. [27]. The results showed that the particulate samples appeared stable at temperatures ranging from 650 ◦C to 1000 ◦C. Solar absorptance decreased due to the particle phase change in which calcium carbonate was transformed into calcium oxide after heating to high temperatures. Aging tests for 500 h at 900 ◦C and thermal cyclic for 1500 cycles between 300 ◦C and 900 ◦C were conducted on silicon carbide (SiC) and CarboHSP® 30/60 by Calderon et al. [28]. Tests to determine solar absorptance, chemical composition, physical properties, and thermal characteristics, as well as morphologic analysis of the samples before and after thermal treatments were performed. The characterization results showed that SiC was more affected in its durability by thermal cycling than by constant-temperature aging treatment, while CarboHSP® was affected by temperature aging rather than thermal cycling. SiC reacted with oxygen to form SiO2 on the surface, with a positive effect on its solar absorptance. Nev- ertheless, with thermal cycles, the SiC particle surface became damaged, and the reaction continued with the additional newly exposed surface. Meanwhile, CarboHSP® reduced its solar absorptance with the high temperatures only due to changes in its surface chemical composition. However, thermal cycling showed no negative effect on the properties of CarboHSP®. Three different granular materials were characterized and tested by Díaz- Heras et al. [29] for use as heat-transfer fluids and storage materials in CSP systems: sand, carbo Accurist ID50, and silicon carbide. Thermal aging at 900 ◦C for 500 h was conducted. The samples were also fluidized for 250 h at twice the minimum fluidization velocity (at ambient temperature). The properties of the particles before and after fluidization and before and after heating to a high temperature (900 ◦C) were investigated. The results showed that there was no significant change in their chemical compositions, and their specific heats and absorptivities did not vary notably after the tests. The main attractive property of SiC and Carbo is their very high absorptivity, which is around or over 0.9, while for the sand it is around 0.4. However, the abundance of sand makes it cheaper thanPDF Image | Low-Cost Particulates Used as Energy Storage and Heat-Transfer Medium
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