Long-term performance of a subway source heat pump system with two types of front-end heat exchangers

The rapid development of subways and resulting deterioration of the thermal environment in tunnels have led to thermal pollution of underground spaces, which is a problem that cannot be ignored. The subway source heat pump system (SSHPS) with a tunnel-lining capillary heat exchanger (CHE) is an effective technology for solving this problem. Existing researches mainly focused on the short-term performance of the SSHPS. Few studies have analyzed the long-term performance of the system under complex periodic boundary conditions. In this study, a simulation model of an SSHPS, which conforms to an actual demonstration project located in Lingshanwei Subway Station of Qingdao Metro Line 13, was built by coupling a flat CHE for the open-cut section and a circular CHE for the concealed excavation section of a subway tunnel. Field test results of the SSHPS performance were compared with those of the simulation. The maximum relative error is 13.96%, which shows that the simulation model has high accuracy and can satisfy the requirements of practical engineering applications. Based on the verified model, the system performance under complex periodic boundary conditions was simulated for 10 consecutive years. The results indicate that the average heat flux of the flat and circular CHEs has little variation during the continuous operation of the system for 10 years, and the variations during the cooling and heating seasons are less than 1.24 W/m2 and 0.36 W/m2, respectively. The annual air temperature increments in the tunnel with flat and circular CHEs are approximately 0.226 °C and 0.324 °C, respectively. The average coefficients of performance of the heat pump unit in the cooling and heating seasons are 4.43 and 5.11, respectively, indicating that the heat pump unit operates efficiently throughout the year. The main reason for the long-term high performance of SSHPS is the annual thermal balance of the subway tunnel. The thermal balance of the subway tunnel during long-term operation should be considered when designing an SSHPS system. This study can provide a reference for optimizing the design and operation of the SSHPS.