Coupling effect between tunnel lining heat exchanger and subway thermal environment

If the waste heat generated during the long-term operation of a subway cannot be effectively eliminated, it will lead to the deterioration of the tunnel thermal environment and thermal pollution of the underground space. A subway source heat pump system (SSHPS) with a tunnel lining heat exchanger is an effective technique for addressing this problem. The present studies mainly focus on the design and performance optimisation of front-end heat exchangers and lack the analysis of the influence of heat exchanger operation on the thermal environment within the tunnels. Based on a demonstration project of an SSHPS in a cold region of China, this study established a coupled heat transfer model involving tunnel air, a lining heat exchanger, and surrounding rock and built a system simulation module in the TRNSYS platform. Based on the validated system simulation module, the coupling effect between the lining heat exchanger operation and the tunnel thermal environment was analysed under different tunnel environment conditions. The results show that when the heat produced in the tunnel increases at the rate of 100 W/m, the heat transfer between the heat exchanger and the tunnel air decreases by 4.35 W/m2 in the cooling season, and increases by 4.36 W/m2 in the heating season. In the cooling season, when the surrounding rock temperature rises by 2 °C, the heat released by the heat exchanger to the surrounding rock decreases by 3.6 W/m2, while that to the tunnel air increases by 0.68 W/m2. The results indicate that the heat generated by subway operations directly affects the heat transfer efficiency between the tunnel lining heat exchanger and tunnel air. The increase in the temperature of the surrounding rock causes the heat exchanger to release more heat to the tunnel air and vice versa. It is suggested that when the subway tunnel heat generation or heat exchanger operation has adverse effects on the tunnel environment, the operating condition of the tunnel heat exchanger should be adjusted accordingly to prevent the tunnel air temperature from exceeding the design limits. This study can provide a reference for the engineering design of subway tunnel lining heat exchangers.