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.

124 transferred to a useful heat carrier. Properly designed packed-bed systems can rival the performance of leading conventional solutions. Theoretically, a counterflow regenerator with a high number of transfer units can approach 100% efficiency (i.e., the exhaust gas temperature approaches that of the cold inlet stream). In practice, however, efficiency is limited by finite system dimensions, heat losses to the environment, and non-ideal flow conditions. For sand-based regenerators used in ventilation or flue gas systems, thermal utilization rates of approximately 60–85% are achievable. For instance, experiments have recorded values around 73% for stone-filled solar air heaters [6], which closely align with theoretical predictions. Particularly impressive results have been achieved in high-temperature applications: the dual-layer radial storage unit developed at KTH demonstrated over 90% heat retention during charge–discharge cycles at temperatures up to 800 °C [10]. Although efficiency tends to be somewhat lower at low temperatures due to relatively larger losses, recent experiments suggest that values exceeding 80–90% are quite feasible with good insulation and optimized layer configurations. CONCLUSIONS Annular channels filled with porous materials like sand offer an effective solution for recovering low-grade waste heat (50–150 °C). These systems combine direct heat exchange with thermal storage, benefiting from the high surface area and heat capacity of the filler. Studies report heat recovery efficiencies of 70–90%, with performance comparable to advanced turbulent heat exchangers [6, 10]. Sand is a low-cost, safe, and durable thermal storage medium, suitable for various applications—ventilation systems, flue gas recovery, and industrial processes. Such heat exchangers can be implemented as compact, stationary modules with minimal maintenance. Current research targets improvements in flow resistance (e.g., radial configurations), particle size optimization, and numerical modeling to further enhance efficiency.