Thermal and Thermomechanical Performance of Energy Piles with Double U-Loop and Spiral Loop Heat Exchangers

摘要

This study investigates the thermal and thermomechanical behavior of two energy piles with either double U-loop or spiral loop heat exchanger configurations. The piles were installed in a sandstone formation, and a heating-recovery-cooling-recovery cycle was applied to the piles. In both piles, the shaft resistance evaluated from the measured axial-strain distributions was greater in the heating mode than that in the cooling mode. In the heating model, radial expansion was detected. The radial stress at the pile-soil interface increased due to the high stiffness of the sandstone, which in turn increased the shaft friction. In the cooling stage, the radial contraction of the pile was measured, which in turn decreased the radial stress at the pile-soil interface. The amount of pile contraction was similar to the thermally induced free contraction, and consequently large pile settlement was measured at the top of the pile during cooling. The normalized heat-transfer rate shows similar thermal efficiency between heating and cooling, indicating the heat transfer was not much affected by the radial expansion and contract of the piles. (C) 2019 American Society of Civil Engineers.