Improved modeling analysis on heat transfer performance of deep coaxial borehole heat exchanger with different operation modes

The deep coaxial borehole heat exchanger (DCBHE) is a highly promising technology which has garnered significant attention. However, there has been a notable scarcity of studies focusing on the physical and mathematical models essential for accurately describing the heat transfer processes involving the DCBHE, well, and rock. This study addresses this gap by employing an improved numerical heat transfer method through programming to conduct an in-depth analysis of the heat transfer performance. The model is solved using the finite volume method (FVM) and validated using experimental data from an existing literature. The simplified effect of pipes and filling material on the heat transfer performance of the DCBHE are analyzed. Furthermore, the characteristics of outlet temperature, heat extraction power, change rate, reduction degree of DCBHE with different operation modes are also analyzed, considering both the heat extraction of one season and 20 years. The results indicate that the simplification of the outer pipe and filling material as thermal resistances is unreasonable while the simplification of inner pipe is acceptable. The maximum outlet water temperature decreases year by year, as the rock-soil temperature fails to recover to its initial level. The water outlet temperature decreases with the run stop ratio while the heat extraction increases. Moreover, the operation mode of 2:2 is recommended both in one heat extraction season and 20 years operation.