Onsite measurement and performance analysis of capillary heat pump system used in subway tunnels

This study proposed a ground source heat pump system using capillary networks as the front-end heat exchanger, to alleviate thermal pollution and utilize waste heat generated in subway tunnels and the surrounding rock. We conducted onsite measurement of the capillary ground source heat pump system based on a real engineering project involving a subway tunnel. Subsequently, the heat transfer performance of the capillary heat exchanger and energy efficiency of the system were analyzed. The heat transfer capacity of the capillary heat exchanger was approximately 45-80 and 100-110 W/m(2) in a test with a duration of 7 h in winter and a test with a duration of 14 h in summer, respectively. The average coefficient of performance of the heat pump unit and coefficient of performance of the system were 3.72 and 3.18, respectively, in winters, and 3.19 and 2.64, respectively, in summers. Under a 4-day test with an intermittent operation of 8 h per day in summer, the performance of the system was improved and remained stable for four days during the test. The average coefficient of performance of the heat pump unit and of the system for intermittent operation were 3.67 and 2.96, respectively, which were larger than those for 14 h continuous operation in summer. In practical applications, intermittent operation or additional auxiliary cooling sources are recommended in summer conditions.