Proceedings of the International scientific and practical conference ―Science, technology and art in global context (July 8-10, 2025) / OP website: www.naukainfo.com. – Dresden, Germany, 2025. - 140 p.

121 For moderate airflow through a sand bed (e.g., 5 mm particles), heat transfer coefficients of 100–150 W/(m²·K) are achievable—comparable to turbulent convection and far higher than laminar flow in smooth pipes. However, increased heat transfer comes with higher hydraulic resistance. Narrow pore channels create pressure losses, especially at higher velocities (Forchheimer regime). Fine particles cause excessive friction; coarse ones reduce surface area. Therefore, particle size and bed height must be optimized for balanced performance [7]. Annular porous beds function as regenerative systems: hot gas heats the packing, and downstream sections extract heat from previously warmed material. In steady counterflow, a thermal gradient is established across the bed. Alternatively, cyclic operation with flow reversal enables charging and discharging phases, as used in compact ceramic recuperators. Both modes benefit from the large surface area and thermal mass of the filler. SAND AND GRANULAR MATERIALS AS THERMAL STORAGE MEDIA The choice of porous filler significantly impacts heat recovery efficiency. Quartz sand and similar inert granular materials are highly promising due to several key advantages: • High heat capacity and thermal stability. Sand has a specific heat capacity of 0.7–0.8 kJ/(kg·K) and bulk density of 1600– 1800 kg/m³, yielding ~1.3×10⁶ J/(m³·K) [8]. It tolerates high temperatures (>1000 °C) without degradation, unlike water or organic fluids. A volume of ~0.2 m³ heated by 100 K can store ~26 MJ (~7.2 kWh), making it suitable for compact thermal storage. • Low cost and wide availability. Sand is abundant and inexpensive, especially compared to specialized fluids or PCMs. Construction-grade sand or gravel makes systems economically attractive. Large-scale projects, like Finland‘s ―sand battery‖ (100 tons storing ~8 MWh at 500 °C), show its scalability and affordability [9].