Practical Design of Earth-Air Heat Exchanger

Abstract

Earth-air heat exchangers (EAHE) are a passive geothermal technology that utilizes the relatively stable subsurface soil temperature to precondition ventilation air for buildings, thereby reducing conventional heating and cooling loads. This study develops a mathematical model for a horizontal EAHE system that explicitly accounts for heat transfer in both insulated and non-insulated descending pipe sections, providing a more realistic representation of practical installation conditions. The model predicts soil temperature distribution, outlet air temperature, and overall thermal efficiency as functions of key design and operating parameters. Experimental validation was conducted using field data from the URAER research facility in Ghardaïa, Algeria. The numerical results show excellent agreement with measurements, with relative errors ranging from 0.48% to 4.28% along the pipe length, all below the 5% threshold. Soil temperature analysis reveals a marked attenuation with depth: surface fluctuations of approximately 13.75 °C are reduced to less than 2 °C at 4 m depth, with a phase lag of up to 2–3 months. Parametric analysis indicates that increasing pipe length from 10 m to 50 m improves thermal efficiency from 35% to over 80%, while higher air velocities (from 1 m/s to 5 m/s) reduce outlet temperature differences by up to 60%. The vertical–horizontal EAHE configuration achieves outlet air temperatures 2–3 °C lower than the horizontal configuration under identical conditions. These findings confirm that EAHE systems can significantly reduce building energy consumption while enhancing indoor thermal comfort in arid climate regions.