Simplified method and numerical simulation analysis of pipe-group long-term heat transfer in deep-ground heat exchangers

Deep-buried pipe-group heat transfer is a primary form of utilizing geothermal energy in the middle and deep ground layers. This study introduces a methodology to simplify the heat transfer process of a pipe-group into a single borehole within a finite computational domain. The proposed methodology allows for rapid and accurate assessment of the heat transfer performance of the pipe-group with coaxial deep-ground heat exchangers (C-DGHEs). The feasibility of this simplified method is supported by numerical validation. Consequently, a 20-year numerical computation is performed for pipe-group models with spacings of 15, 20, 30, and 40 m, alongside a comparative study on the heat transfer of a single borehole C-DGHE with a computational domain radius of 200 m. For a pipe-group spacing of 15 m, the results demonstrated that the heat transfer boundary is exceeded in the first year of heat extraction but stabilized into a steady decline. For a spacing of 40 m, the decrease in heat transfer over 20 years was less than 15 % compared to the single C-DGHE. This research effectively evaluates the heat transfer performance of pipe groups, providing theoretical support and technical guidance to develop and promote the applications of deep geothermal energy.