Numerical study on thermal storage and exothermic characteristics of subway station fresh air shaft surrounding rock

Subway stations are typically equipped with fresh air shafts to introduce outdoor air, while the surrounding rock and air channels form a vertical earth-air heat exchanger (VEAHE). This study is the first to analyze the year-round thermal storage and exothermic characteristics of the surrounding rock. A numerical model was developed, incorporating ground surface heat transfer and axial heat transfer within the rock. The simulation results aligned well with field test data. This model was then utilized to explore the thermal storage and release properties of the surrounding rock. The findings revealed that both z-directional and r-directional annual temperature wave affect the enclosing rock. The monthly average natural energy efficiency (NEE) and storage efficiency (SE) were determined to be 10.75% and 17.47%, respectively. Notably, the shallow rock exhibited the highest NEE but the lowest SE. The energy used for fresh air pre-treatment derives not only from the rock's current annual cycle but also from previously stored energy. Lastly, λsr and c were identified as major influencers of NEE, while λsr, c, and the burial depth of the shaft collectively impact the air pre-treatment process in the shaft. The findings contribute to further understanding of the thermal process in VEAHE systems.