Enhanced pre-cooling for underground pipe gallery ventilation via a hybrid borehole heat exchanger-capillary ceiling system

The underground pipe gallery ventilation system effectively provides pre-cooling during the cooling season. However, its shallow burial depth limits the long-term stability of heat exchange. To overcome this limitation, a novel hybrid ventilation system was developed, integrating a borehole heat exchanger with a capillary mat–coupled radiant ceiling cooling system. A full-scale experimental platform was constructed to evaluate its climate adaptability and cooling enhancement mechanisms under real operating conditions. Results showed that the radiant ceiling cooling system compensated for the limited cooling capacity of the shallow overlying soil, reducing vertical temperature stratification from 5.1 °C to 3.7 °C compared with a system without ceiling cooling. Fan-assisted ventilation effectively suppressed reverse airflow. The integrated system achieved a maximum outdoor air temperature reduction of 10.8 °C and a peak total heat transfer rate of 6.4 kW, representing a 68.7% improvement over mechanical ventilation alone. In addition, the vertical temperature difference was further reduced to 2.1 °C, leading to a 41.8% increase in heat exchange efficiency compared with non-integrated operation. The system maintained lower indoor temperatures for at least 36.9% of the summer operating period, demonstrating a practical, energy-efficient, and scalable retrofit strategy for existing underground pipe galleries.