Long-term performance of a front-end capillary heat exchanger for a metro source heat pump system

•Analyzed the performance of tunnel lining exchanger under periodic boundary conditions.•A distinct ‘inertia’ phenomenon is observed for the temperature wave in the tunnel surrounding rock.•A critical heat transfer point exists along the CHE tube length.•Recommended CHE tube length was determined. In recent years, the number of subways has rapidly increased, as they can effectively alleviate urban traffic congestion. However, subways can also result in thermal pollution problems in underground spaces. The long-term operation of a subway causes thermal accumulation in the surrounding rock of a tunnel, severely affecting the safe and efficient operation of the subway. The metro source heat pump technology is an effective approach to address the foregoing problem. However, currently, the research on its systematic design methods remains inadequate. In particular, studies on the evolution law of heat transfer characteristics of its front-end heat exchangers during years of operation are limited. In this study, a numerical model of a front-end capillary heat exchanger (CHE) is formulated. The model is based on a practical subway source heat pump demonstration project located in a cold area in China. The annual heat transfer characteristics of the CHE under periodic boundary conditions are simulated and analysed. The results reveal that the temperature of the CHE and its vicinity fluctuates periodically and increases annually under typical design conditions. The fluctuation range of CHE wall temperature gradually decreases from 25.0 °C at 0 m from the CHE inlet to 9.6 °C at 10 m from the CHE inlet in the 10th year. A distinct ‘inertia’ phenomenon is observed when the temperature wave is transmitted to the rock surrounding the tunnel. With the increase in the surrounding rock depth from 0 to 25 m, the amplitude of temperature wave gradually decreases from 4.277 to 0.013 °C, and the reduction rate gradually declines from 24 % to 14 %. Meanwhile, the phase delay of temperature wave increased from 0° to 282.40°. Moreover, a critical heat transfer point exists along the CHE tube length, and the heat transfer law and efficiency before and after the critical point differ. Statistics show that the ratio of CHE average heat flux after the critical point to that before the critical point ranges from 1.11 % to 46.16 % in the heating season and from 30.19 % to 46.12 % in the cooling season, respectively. The recommended CHE tube length is determined to be 4.9–6.3 m based on