Thermal behavior of a novel vertical coaxial geothermal ventilation system under different operating conditions: An experimental study

With the increasing energy consumption in buildings, developing efficient and low-energy ventilation technologies has become essential. The earth-to-air heat exchanger (EAHE) systems utilize relatively stable soil temperatures to precondition outdoor air and reduce building cooling and heating loads. However, conventional horizontal EAHE systems suffer from limitations such as large land occupation, condensate drainage difficulties, and shallow pipe burial depth. To address these issues, a novel vertical coaxial pipe earth-to-air heat exchanger (VCP-EAHE) is proposed and experimentally investigated using a scaled sandbox setup under different soil moisture contents and seasonal conditions in Changsha. The results show that under summer conditions, the system provides stable precooling performance with a maximum inlet–outlet temperature difference of 1.68 °C. Higher soil moisture enhances thermal buffering capacity, reducing the standard deviation of the temperature difference from 0.190 to 0.158. During the transition season, the system exhibits bidirectional thermal regulation, achieving a maximum cooling of 1.31 °C and heating of 0.78 °C. Overall, the VCP-EAHE demonstrates effective thermal regulation and stable operation, providing experimental support for the design and application of vertical geothermal ventilation systems.