Abstract

Packed-bed thermal energy storage (PBTES) using industrial by-products, such as copper slag, provides a costeffective and high-temperature solution for integrating variable renewables and decarbonizing industrial heat. This study presents a novel experimental comparison between axial (A-PBTES) and radial (R-PBTES) PBTES configurations with copper slag as the storage medium and air as the heat transfer fluid. Duplicate tests considered 1.5 h and 3.0 h charging intervals and airflow temperatures of 150 degrees C and 300 degrees C. Because the axial and radial units correspond to different bed volumes, geometries, and porosities, the reported results should be interpreted as a prototype-level comparison under common boundary conditions rather than a one-to-one topology ranking. The temperature profiles obtained from the A-PBTES revealed a clear axial thermocline. The average energy storage capacity achieved was 2.2 +/- 0.7 kWh, and the discharge energy delivered to the airflow averaged 2.1 +/- 0.7 kWh, resulting in a round-trip thermal efficiency of 93.5 +/- 1.5%. The R-PBTES exhibited peak temperatures in the central region, with azimuthal asymmetry decreasing over time. It averaged an energy storage capacity of 0.7 +/- 0.2 kWh and delivered 0.5 +/- 0.2 kWh during discharge, resulting in a thermal efficiency of 73.9 +/- 0.7%. It also showed faster energy exchange and reduced pressure drop. Finally, the thermal power profiles showed that the R-PBTES delivers and stores thermal energy faster than the A-PBTES. The latter configuration also maintains superior thermal stratification, achieving up to 14.5 % higher specific stored exergy compared to the R-PBTES, which experiences higher thermal dispersion. These findings confirm copper slag as a technically viable and low-cost TES material and highlight the radial topology's potential for reduced pressure drops and self-insulation, supporting slag-based PBTES for cost-effective, scalable applications in high-temperature energy storage, industrial process heat, and waste-heat recovery.