Development of novel high-absorptivity darkened sands for high-temperature solar thermal applications

This study introduces a novel method to enhance the solar absorptivity of silica sand (SiO 2 ) for high-temperature concentrating solar applications by diffusing Mn ions into the crystal lattice. The proposed methodology improves previous works, which only coated with a layer around the particle that can be easily removed. The darkened sand was synthesized by mixing silica sand with manganese carbonate (MnCO 3 ) and thermally treating it at temperatures ranging from 600 to 900 °C with varying MnCO 3 concentrations (1:10 to 1:50). XRD, SEM, and EDS analyses confirmed Mn-ion diffusion, resulting in improved optical properties and thermal stability. Experimental results in a directly irradiated solar-particle receiver demonstrate that the darkened sand achieved peak charging efficiencies of 0.59-0.61 (values similar to other high-absorptivity and high-cost materials, such as SiC), representing a 20%–24% improvement over untreated sand, while maintaining mechanical and optical integrity after various cycles of heating/cooling. The enhanced absorptivity significantly reduces the solar flux requirement for high-temperature operation, with system-level models predicting energy input savings of 40%–60% for a bed at 1000K. This approach offers a durable solution for improving the performance of solid particle receivers in Concentrating Solar Power systems. • Mn ions diffusion into silica sand lattice for permanent coating. • The darkened sand achieves optical properties comparable to silicon carbide. • A complete material characterization has been performed: XRD/SEM/EDS/colorimetry. • Proven performance and operational durability under fluidized bed conditions. • Charging efficiencies increase a 20%–24% over raw sand.