Seasonal Response of Energy Foundations During Building Operation

This paper focuses on the response of two full-scale energy foundations beneath an 8-story building during operation of a heat pump over a 658-day period. During circulation of fluid having temperatures ranging from 7 to 35 °C through the closed-loop heat exchangers within the foundations, the temperature of the reinforced concrete ranged from 9 to 30 °C and was relatively uniform with depth. Estimates of the average heat exchange per unit meter ranged from 91 to 95 W/m. The thermal axial strains during the first year of heating and cooling were elastic and recoverable, but a change in mobilized coefficient of thermal expansion occurred in the second year, potentially due to changes in interface shear stresses. The smallest magnitudes of thermal axial strains were observed at the top and bottom of the foundations due to the restraint provided by the overlying building and underlying bedrock. Issues were encountered in the interpretation of the thermal axial stresses, and were attributed to thermally induced dragdown and transient differences in temperature between the reinforced concrete and sensors. The maximum thermo-mechanical axial stress in the foundations was approximately 10 MPa, well within structural limits. The mobilized side shear stresses follow a nonlinear profile with depth, potentially due to the combined effects of thermal expansion and downdrag. The thermal axial displacements estimated at the foundation head relative to the toe ranged from −1.5 upward to 0.8 mm downward during heating and cooling of the foundation, respectively, which are not expected to affect the building.