Study On Synergistic Characteristics Of Total Thermal Process Of Subway Source Heat Pump System

In recent years,the construction and operation of urban subway systems have grown rapidly.While they have helped alleviate urban traffic congestion,they also face the problems of high energy consumption and increasingly deteriorating thermal environments.The subway-source heat pump system,which utilizes the front-end heat exchangers of the tunnel lining,is an effective solution to these problems.However,the current design and operation of the system are based on traditional air conditioning systems,leading to a mismatch in system sizes and increased investment and operating costs.The differences in the main subsystems’ resistance-capacitance(RC)in the subwaysource heat pump system cause asynchronous energy flow responses,and understanding the system’s synergistic properties is important for designing and energy-efficient operation.This study investigates the synergistic properties of the full thermal process of the subway-source heat pump system.State-space dynamic RC models of the front-end heat exchanger,heat pump,and end-building subsystems were established and their accuracy was verified through on-site testing.The key factors influencing the system’s overall thermal performance were determined using orthogonal experimental methods,and the coupling relationship between key factors and system RC characteristics was analyzed.The synergistic properties of the system’s energy flow under multi-frequency response were also explored.The main conclusions under the parameters set in this study are as follows:(1)The daily system coefficient of performance(COP)is significantly affected by the specific heat capacity and thermal conductivity of the tunnel lining and buried pipe depth,while the annual system COP is significantly affected by the specific heat capacity and thermal conductivity of the surrounding rock.These results suggest that the RC parameters of the subway-source heat pump system are critical factors affecting its operating performance.(2)The variation of the thermal resistance and capacitance parameters on the system source side resulted in the following thermal response pattern: when the lining thermal conductivity was between 1.5 W/(m·K)and 3.5 W/(m·K),the mean system source heat flux increased by 24.74%,and the response time was delayed by 1.32 h,with an increase of 114.55% in the decrement factor.When the lining specific heat capacity was between1.5 MJ/(m~3·K)and 3.5 MJ/(m~3·K),the mean heat flux remained the same,but the response time was increased by 1.4 h,with a decrease of 48.06% in the decrement factor.When the burial depth of the heat exchanger was between 0.1 m and 0.4 m,the response time was increased by 6.72 h,with a decrease of 96.74% in the decrement factor.Therefore,it can be concluded that the response time of the thermal flux increases with the increase of lining specific heat capacity and burial depth,and decreases with the increase of lining thermal conductivity;the decrement factor of the heat flux decreases with the increase of lining specific heat capacity and burial depth,and increases with the increase of lining thermal conductivity.The system’s thermal flow delay time increases with increasing specific heat capacity and buried pipe depth,and decreases with increasing thermal conductivity of the tunnel lining.The amplitude decrement factor of the thermal flow decreases with increasing specific heat capacity and buried pipe depth and increases with increasing thermal conductivity of the tunnel lining.(3)The heat source inside the tunnel,outdoor temperature,and system operating mode can all affect the coordination between the heat flow on the system source side and the load side.Due to the RC characteristics of the system’s main body,the high-frequency parts of temperature and thermal flow waves in the external environment are similar to fast relaxation variables in the system’s synergy,while the low-frequency parts are similar to slow relaxation variables.Among the factors influencing the system’s thermal flow synergistic properties,the high-frequency thermal disturbance has a smaller impact and can be eliminated,while the low-frequency thermal disturbance is the main influencing factor.This study explores the asynchronous energy flow changes caused by the differences in the main subsystems’ RC characteristics in the subway-source heat pump system,providing a theoretical basis for reducing the initial investment and improving the operating efficiency of the system.