Heat Transfer Model of the Front-end Capillary Heat Exchanger of a Subway Source Heat Pump System

Abstract The use of metro source heat pumps can help alleviate the deterioration of the subway thermal environment and convert the “waste heat” into a valuable resource for heating aboveground buildings. At present, this technique represents a highly effective energy-saving method to control the thermal environment in subway tunnels. In this study, the front-end capillary heat exchanger (CHE) of the subway source heat pump system of a demonstration project in Qingdao was considered as the research object. Considering a CHE that was installed in a multilayer circular composite medium, a fluid-thermal coupled heat transfer model was established by combining theoretical analyses and numerical simulation techniques. A module pertaining to the CHE was implemented on the TRNSYS platform based on the corresponding fluid-thermal coupled heat transfer model. The field test results for the performance of the subway tunnel CHE system were compared with the simulation results of the module. The maximum and average relative errors were 9.3% and 3.5%, respectively. The verification results indicated that the developed simulation module is highly accurate and can satisfy the requirements of practical engineering applications. Based on the validated module, the influence of three typical design factors (inlet water temperature, flow velocity and type of surrounding rock) on the heat transfer performance of CHE was analyzed. This study can provide a theoretical basis for the design and operation optimization of front-end CHEs of metro source heat pump systems.