Performance Analysis of a Hybrid Subway Source Heat Pump System Using Capillary Heat Exchanger

A large amount of free heat generated by the running of subway is released to the tunnel. Due to arc structure of subway tunnels, conventional ground source heat pumps and large-diameter heat exchangers are difficult to efficiently extract the heat and maintain surrounding-rock thermal balance. To sustainably take advantage of these free heat, in this paper, a hybrid subway source heat pump system is proposed, which is integrated by subway source heat pump system, 4.3 mm diameter capillary heat exchanger and auxiliary cooling system. A dynamic simulation model of the hybrid heat pump system, which includes the heat transfer process between capillary heat exchanger and surrounding-rock was developed on TRNSYS. Field tests were conducted at a subway station of Qingdao (China) to validate this model. Under different strategies, the coefficient of performance of the hybrid heat pump system and characteristics of surrounding-rock temperature field are compared. The results indicated that the hybrid heat pump system can ensure appropriate surrounding-rock temperature without overheating (from 33.9 degrees C to 31.3 degrees C). Additionally, the coefficient of performance of the subway source heat pump units was 4.432, increased by 2.52%, the energy consumption of the subway source heat pump system can be decreased by 5.55%.