INVESTIGATION OF THE CONCENTRIC VERTICAL GROUND LOOP OF A HEAT PUMP IN AN ENERGY MICRO-PILE-Paper

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The study numerically evaluated heat transfer in vertical ground-loop heat exchangers and assessed a concentric-pipe design integrated into an energy micropile. Using ANSYS Fluent, the researchers compared U-tube, concentric, and spiral configurations with equal total pipe lengths. The model was validated against published experimental data; agreement was better in summer than winter.<br /><br />The U-tube produced the greatest overall heat transfer, about 60% more than the concentric design and 25% more than the spiral design. Although the concentric configuration had the lowest total temperature rise, its annular flow section showed a greater temperature rise per unit length than the corresponding sections of the other configurations. The authors note that concentric piping may be easier and less costly to incorporate into hollow-bar micropiles, potentially avoiding low-conductivity HDPE piping if the steel pile can serve as part of the heat exchanger.<br /><br />A second simulation modelled a 60-m-deep energy micropile with fluid flowing downward through an inner HDPE pipe and upward through the surrounding annulus. With a measured ground-temperature profile applied, the fluid warmed by 6.3°C, transferring 12.0 kW. The annular section accounted for 5.08°C of the rise and 9.68 kW of heat transfer—about 4.2 times the inner pipe’s contribution. Temperature increased semi-linearly along both flow paths, with distinct gradients in the inner and annular regions.<br /><br />The results suggest that conventional U-tubes offer higher overall performance, while concentric systems may be attractive for structural integration into micropiles. Further research is needed, particularly to better account for changes in soil temperature around the pile.

Keywords

ground-loop heat exchanger

vertical heat transfer

energy micropile

concentric-pipe design

U-tube configuration

spiral configuration

ANSYS Fluent

annular flow

thermal performance

soil temperature profile