Parameterization of a calibrated geothermal energy pile model

Abstract

This paper describes the calibration and parameterization of a numerical model for conductive heat transfer from a group of geothermal energy piles into the soil surrounding the piles. Calibration was performed using Thermal Response Test (TRT) data collected from a group of full-scale in-situ geothermal energy piles in Colorado Springs, CO. The calibration of the three dimensional model incorporated field data to represent boundary conditions including inlet temperature, atmospheric temperature, and subsurface temperatures at different locations within the pile group. Following calibration, the model was parameterized to understand the role of heat exchanger configuration with a given energy pile as well as the role of pile spacing in an energy pile group. Parametric combinations were compared using heat transfer per unit length of the energy pile (W/m). The results of the parametric study indicate that heat transfer increases by up to 8% for an even heat exchanger layout compared to an uneven layout when considering a 15.2 m long, 0.61 m ⌀ energy pile configured with a W-shape heat exchanger. These results also provide useful insight into the cross-sectional temperature distribution of the aforementioned energy pile configuration. Energy pile temperature was observed to vary by up to 20% across the core of the pile during heating for various heat exchanger layouts. This uneven temperature distribution may have implications on the estimation of in-situ thermal axial stresses in energy piles. Specifically, using measurements at strain gage locations may underestimate thermal axial stress during heating.