Understanding transient heat transfer in large-scale ground heat exchanger (GHE) matrices: Insights from high-resolution analytical solutions

Abstract

This paper reports on a set of high-resolution analytical solutions, which combines a composite-medium line-source solution, the conventional finite and infinite line-source solutions, and a quasi-3D model for fluid inside U-shaped tubes. This model extends our full-scale line-source model to deal with the thermal interaction between adjacent boreholes and the vertical variation in fluid temperature along U-shaped tubes. Based on this model, we analyze both hourly high-frequency temperature responses and long-term temperature evolution of GHEs clusters. The results indicate that 1) the heat capacity of backfilling material can heavily influence the prediction of temperature extrema and 2) the mainstream of heat flux within GHE matrices shifts gradually from the horizontal direction to the vertical direction due to the thermal interaction between GHEs. Finally, this paper suggests a two-borehole approach to approximating the average temperatures of large-scale GHEs matrices.