Study On The Heat Transfer Characteristics Of Ground Heat Exchangers For Ground-source Heat Pumps

Ground source heat pump (GSHP) system is one of the most promising innovative air-conditioning technology aimed at energy conservation and environmental protection, and is one of the most advanced tasks in international HVAC industry. One of the key techniques for GSHP popularization and application is to perfect heat transfer models for the ground heat exchangers to simulate the actual heat exchange process. State-of-the-art research situation at home and abroad and several typical heat transfer models of vertical heat exchangers are summarized in this paper. In allusion to the slate geological characteristics, the heat transfer characteristics of the soil temperature field around the vertical U-tube for GSHP are analyzed.The characteristics of soil temperature distribution are studied at the beginning of the paper, and the computational model of original soil temperature field distribution is established. For Dalian district, the temperature within 75m underground is simulated and the results are accorded with experimental data. Then, a two-dimensional model of the heat transfer around a single vertical U-tube for GSHP is established and the exit water temperature and the soil temperature distribution around the buried pipe running in winter are simulated, and the results are tallied with experimental data. In addition, the influencing factors, such as the backfill thermal resistance, the pipe thermal resistance, the flux in the pipe and different operation modes, which affect the heat transfer performance of the buried pipe, are analyzed in theory and the thermal influencing radius based on the soil conditions in Dalian is computed. Next, a transient three-dimensional heat transfer model of a single vertical U-tube is also established and the soil temperature distribution around the buried pipe running in different operation modes in summer are simulated and the results are consistent with experimental data. Finally, the simulated results of the different models are compared and came to the corresponding conclusions. The intermittent operation mode is favorable for the recovery of soil temperature. Through an intermittent operation, the soil temperature is recovered, and the output coefficient of geothermal energy is improved, and the number of boreholes is decreased, and thus the heat exchange ability of the geothermal heat exchange system is exerted furthest.