Heat extraction performance analysis of the multi-level spiral pipe U-shaped borehole heat exchanger under different design parameters

This study proposes a multi-level spiral pipe U-shaped borehole heat exchanger (MLS-U-BHE) to enhance geothermal energy utilization by replacing equal-diameter straight pipes with spiral pipes in horizontal wells. MLS-U-BHE extends fluid heating length without increasing number of boreholes, reducing drilling costs compared to traditional U-BHEs (T-U-BHE) with multi-level. A coupled heat transfer numerical model between BHEs and the reservoir was developed and validated by experimental and literature data. Using the single-level spiral pipe U-BHE (SLS-U-BHE) as a base model, effects of operating parameters, horizontal well length, number of levels, spiral coil radius ( r ), pitch ( h ) and vertical spacing on heat extraction power (HEP) were analyzed. Results show stable long-term operation and recommend spacing above 73.07 m for multiple units. Spiral pipe U-BHE outperforms T-U-BHE under various conditions. The optimal SLS-U-BHE parameters with the maximum effective heat extraction power are r = 0.2 m and h = 0.2 m, which improve outlet temperature and HEP by 6.48% and 13.58%, respectively. Variation in r affects heat extraction performance more than h . HEP value of MLS-U-BHE (with five levels) is 2.67 times that of T-U-BHE, with a 48.39% rise in outlet temperature. 70 m vertical spacing for three levels has maximum HEP. New design offers a valuable guidance for future U-BHEs development.