Low‐Temperature Experimental Characterization of Magnetite‐Based Rock‐Bed Thermal Battery

Rock bed thermal energy storage (RTES) systems are increasingly used as affordable and scalable solutions for low‐temperature heat storage. However, experimental investigations of magnetite packed beds under repeated charge–discharge cycles remain limited. This study experimentally evaluates the thermal performance of a horizontal cylindrical RTES system filled with magnetite and operated as a low‐temperature thermal battery for greenhouse applications. Two airflow strategies are compared over two successive cycles of charging and discharging: (i) unidirectional airflow during both charging and discharging (Model 1) and (ii) reversed airflow during discharge (Model 2). Thermal performance is analyzed through temperature stratification, heat transfer dynamics, energy recovery, and dimensionless performance indicators, including the Thermal Efficiency Index ( TEI ), Temperature Uniformity Index ( TUI ), Normalized Temperature Standard Deviation ( NTSD ), and Temperature Gradient Ratio ( TGR ). Results show that airflow reversal significantly enhances thermal propagation and heat extraction. Model 2 achieves a more pronounced thermal‐front response, generally higher TUI and TGR values, and higher energy recovery, delivering up to 30% more recovered heat during the first discharge period and approximately 15.79% more during the second discharge period compared with Model 1. Moreover, Model 2 exhibits higher TEI values across the investigated operating periods, with particularly noticeable advantages during the first discharge and second charging periods. These findings demonstrate that discharge flow reversal is an effective operating strategy for improving the responsiveness and heat‐recovery performance of low‐temperature magnetite RTES systems.