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.

123 mimics a thermal wheel but without moving parts. Compact ceramic regenerators already reach ~80–90% efficiency [4]; sand offers a cheaper and flexible alternative. With its large surface area and thermal inertia, sand enables recovery efficiencies of 70–85% and smooths out temperature fluctuations. Moisture recovery is also possible: condensation during the cool phase can later evaporate during flow reversal, enabling both sensible and latent heat exchange—especially when hygroscopic or moistened sand is used. 2. Gas Boiler Flue Systems In boilers, flue gases (120–180 °C) carry substantial heat loss. A coaxial heat exchanger with a sand-filled annular gap can act as a packed-bed economizer. Flue gas heats the inner pipe, which warms the adjacent sand layer. A cold fluid outside the casing then absorbs the stored heat. Sand smooths heat fluctuations during burner cycles and improves boiler efficiency by 5–10%, lowering exhaust temperature by ~30–50 °C. The annular gap must be kept thin to compensate for sand‘s low thermal conductivity; metal additives (e.g., shavings) may enhance performance. Sand is inert to flue gas byproducts and more corrosion-resistant than aluminum. Condensate drainage can be integrated if needed. 3. Kitchen and Bathroom Exhaust Though temperatures are low (~20–40 °C), exhaust air volumes are significant. A sand-filled regenerator integrated into ventilation ducts can recover part of this energy. Two alternating channels transfer heat from exhaust to supply air via flow reversal. Sand also acts as a filter, capturing grease and particulates. Recovery efficiency is ~50–60%—still beneficial for energy savings. Additionally, the granular bed dampens airflow noise, improving acoustic comfort. THERMAL EFFICIENCY The key performance metric of a heat recovery system is the heat recovery efficiency—the fraction of thermal power extracted from the exhaust stream and