Comparative analysis of heat transfer performance of coaxial pipe and U-type deep borehole heat exchangers

This paper discusses the advantages of medium-deep geothermal energy and describes the structural principles of coaxial pipe and U-type deep borehole heat exchangers (BHEs). The heat transfer performances of the two types of deep BHEs are compared. The preconditions and heat transfer mechanism are explained to reveal the substance of heat transmission. Energy equations are established for circulating fluid, grout, steel pipe and underground media, and the implications of related parameters are discussed. The heat transfer performances of deep BHEs are analyzed, the heat extractions and the temperatures of circulating fluid and rock soils are investigated, and related elements influencing these parameters are explored. Therefore, the changes of heat absorption or temperatures with the correlative elements are shown, and comparisons are made to demonstrate the differences of the two types of deep BHEs. The heat transfer performance of a U-type deep BHE is better than that of a coaxial pipe deep BHE. The comparative analysis of coaxial pipe and U-type deep BHEs enables the selection of BHEs according to a project's actual situation. This is beneficial to the rational utilization of medium-deep geothermal energy, and it promotes the development of heating technology used in geothermal energy applications.