Performance of a subway source heat pump demonstration project during heating season

The prolonged operation of subway systems can result in thermal pollution within underground spaces. The subway source heat pump system (SSHPS) represents one of the effective technological solutions to address this issue. Nevertheless, existing research exhibits a scarcity of studies focusing on the long-term operational performance of this technology in real-world engineering applications. Drawing upon a demonstration project of a SSHPS implemented in a cold region, this study evaluated the long-term operational characteristics of the system during the heating season. The results indicate that during the testing period, the coefficient of performance (COP) of the heat pump unit was 8.1, while the system-level COP and heating seasonal performance factor (HSPF) of the system were 4.9 and 4.3, respectively. Notably, the HSPF exceeded the first-level energy efficiency standard specified in national regulations by 13 %, thereby clearly demonstrating the high efficiency of this system. However, due to variations in operating conditions, the unit area heat exchange capacity of the tunnel lining heat exchanger fluctuated significantly during the testing period, ranging from 26.7 to 86.9 W/m2. Additionally, during system operation, issues such as frequent start-stop cycles of the heat pump unit and high energy consumption of the fluid distribution system were identified. The analysis reveals that adopting a strategy with a large temperature difference and low flow rate can significantly reduce the energy consumption of the distribution system while decreasing the start-stop frequency of the heat pump unit. This method not only improves system efficiency but also ensures stable and reliable operation. Furthermore, this study offers theoretical guidance for the efficient operation of SSHPS in practical engineering.