Experimental research on the heat balance of independent heat extraction-release double helix energy pile

The use of spiral buried pipe of ground source heat pump in civil defense engineering shows great potential, exploring its complex heat transfer features and thermal balance through experimental approaches is essential. This paper utilizes an independent heat extraction-release double helix energy pile to manage condensation heat discharge in underground projects. An experimental platform was established to compare three scenarios: pure heat release, centralized heat extraction, and uniform heat extraction. The pure heat release mode resulted in substantial soil heat accumulation, whereas the centralized heat extraction and uniform heat extraction demonstrated significant reductions in average temperature rise, with decreases of 24.7 % and 31.3 % near the buried pipes, and 27.3 % and 46.3 % in the central regions. The heat-extracting pipe under centralized heat extraction and uniform heat extraction conditions remove 18.7 MJ and 20.3 MJ of accumulated heat, amounting to 10.3 % and 11.2 % of the total heat dissipation, respectively, effectively mitigating the thermal accumulation in the soil during operation. Besides, the outlet temperature of the heat-extracting pipe is predominantly influenced by the highest temperature in the heat exchange zone, lowering the inlet temperature effectively increases the temperature difference between the inlet and outlet, enhancing the amount of heat removed. The accumulation of underground heat can enhance the heat extraction performance of the pipes, and increasing the water flow rate of the heat-releasing pipe positively impacts the heat extraction capacity. After soil temperatures rise, running the heat-extracting pipe with cooler inlet water and a moderately higher flow rate is advised for extracting the accumulated heat.