Evaluation of spherical ceramic particles for solar thermal transfer media

Two ceramic materials, spheroidized sintered bauxite and fused zircon, were evaluated for potential use as thermal transfer media in an advanced high temperature solid particle solar receiver. Both materials were sufficiently resistant to aggregation via sintering to permit repeated cycling of the particle charge to temperatures approaching 1400C provided no pressure was applied to the particle bed. Application of pressure dramatically enhanced sintering above 1100C and yielded non-flowable sintered particle masses. This indicates that moving particle bed storage configurations must be employed for temperatures in excess of 1100C. The sintered bauxite material exhibited a higher solar absorptance (0.94) than the fused zircon (0.74) for all conditions investigated. Extended thermal treatments in air markedly degraded the solar absorptance of both materials. Degradation was attributed to modifications in crystalline phase assemblages and distribution of multi-valent iron and titanium cations. High solar absorptances could be restored by reheating the oxidized specimens in mildly reducing atmospheres. Potential improvements in the solar absorptance of the fused zircon material may be possible through doping the material with higher concentrations of transition metal and rare Earth cations.