Packed‐bed thermal storage solar air collectors with porous media: Performance influencing parameters and enhancement strategies

Abstract Solar air heaters (SAHs) are promising low‐cost solar thermal systems for heating and drying applications; however, their performance is limited by the low convective heat transfer coefficient between the absorber and the flowing air. Double‐pass configurations incorporating porous media have therefore attracted increasing attention because they enhance airflow mixing and heat transfer. Nevertheless, existing reviews mainly focus on individual enhancement techniques or thermal efficiency, with limited attention to the combined effects of porous‐medium characteristics, airflow configurations, operating conditions, and associated hydraulic penalties. To address this gap, this review provides an integrated critical and quantitative assessment of double‐pass SAHs, emphasizing porous‐media and packed‐bed configurations and the trade‐off between thermal enhancement and pressure loss. The literature was systematically analyzed by comparing collector configurations, porous‐media characteristics, operating and meteorological parameters, enhancement strategies, experimental testing, and CFD investigations. The reviewed studies demonstrate that porous media, fins, artificial roughness, and optimized airflow configurations can substantially enhance heat transfer, with thermal efficiencies reaching approximately 87% in advanced systems. However, excessive turbulence and flow obstructions can significantly increase pressure drop and pumping power requirements. High‐porosity media and optimized airflow arrangements generally provide a more favorable thermohydraulic compromise. The review highlights the need for standardized thermohydraulic evaluation, systematic optimization of porous‐media properties and airflow conditions, long‐term outdoor validation, and integrated experimental–CFD investigations.