Numerical analysis of the effect of inner pipe thermal conductivity on heat transfer in deep borehole heat exchangers

Geothermal energy, recognized as a clean and sustainable solution, has become crucial to global energy transformation. This study focuses on a coaxial-type deep borehole heat exchanger geothermal utilization system in Xi’an to establish a comprehensive numerical model to investigate the effects of varying thermal conductivity and insulation depths of the inner pipe on the heat transfer performance of a deep-buried pipe. The results showed that the heat transfer rate of the buried pipe under different water temperature conditions is approximately 16 % higher with an insulated steel inner pipe than with a PE pipe. When the depth of the insulated steel pipe reaches 600 m, despite the insulation depth increasing by only 24.00 %, the heat loss rate of the buried pipe is significantly reduced to 43.07 %, which shows that an insulated inner pipe to a specific depth can effectively enhance the heat transfer of the buried pipe. This study presents a novel approach to enhance heat transfer in coaxial-type deep-buried pipes by focusing on inner pipe material selection and segmented thermal insulation. The findings offer valuable guidance on the design of deep-buried pipe heat transfer systems.