Performance evaluation of an earth–air heat exchanger system for temperature and humidity regulation in underground granaries

Underground grain storage facilities benefit from the thermal inertia of surrounding soil, but grain respiration and coupled heat and moisture transfer may induce condensation and mold growth, threatening storage safety. This study experimentally compared conventional mechanical ventilation (CMV) and an earth–air heat exchanger (EAHE) system for temperature and humidity regulation in an underground wheat granary in summer. Seven cases were conducted in a small-scale underground granary, including static storage, CMV, and EAHE, with airflow rate and ventilation duration as variables. Twenty sensors measuring temperature and relative humidity were installed on the central vertical plane of the grain bulk, and four additional sensors monitored the headspace, ambient air, and air conditions in the ventilation system. Temperature and relative humidity distributions, field uniformity based on relative standard deviation (RSD), condensation safety margin, and energy performance were evaluated. The results show that CMV and EAHE differed in hygrothermal regulation. Although the cooling effect of CMV improved with increasing airflow rate, high airflow rates promoted non-uniform airflow distribution and heat accumulation in the central region, forming a hot core. In contrast, EAHE suppressed hot-core formation by preconditioning the inlet air through soil. After 12 h at a high airflow rate of 198 m 3 /h, the temperature field RSD of EAHE decreased to 1.73%, approximately 54% lower than that under CMV. At the monitored locations and times, the local condensation safety margin remained positive, with a minimum of 5.4 °C, indicating that the local intergranular air remained unsaturated. The EAHE exhibited smaller temporal variations in the condensation safety margin during ventilation. Energy analysis further indicated that EAHE achieved a fan-based COP of 14.6 and an exergy efficiency of 32.9%, demonstrating its potential as an energy-efficient ventilation strategy for underground grain storage.