Simplified two-step, cross-sectional approach for coupled heat and moisture transfer modeling of shallow, horizontal, ground-based heat Exchangers

This study evaluates a two-step, cross-sectional approach for designing shallow, unsaturated, horizontal, ground-based heat exchangers (GHXs). Numerical modeling was conducted for coupled heat and moisture transfer around GHXs under transient climatological conditions through a cooling season. Soil samples were collected in Texas and Wisconsin to measure thermal conductivity dry-out curves and soil-water characteristic curves for use in the modeling. Average daily meteorological data from central Texas was applied for the top boundary condition. Heat generation from the GHX was calculated by total condenser heat rejection from an environmental control unit based on ambient temperatures and unit specifications. In the first step of the modeling, results showed that the fluid temperature through the GHX loop was approximately 43 ºC, and rapid heat and moisture fluxes were observed around the GHX loops. High moisture flux along the upper surface was also observed due to high ambient temperatures that occur during the summer season. Using these results, exiting temperature of the GHX was estimated for the second cross-sectional modeling step. This two-step, cross-sectional modeling approach provides a systematic analysis of coupled heat and moisture transfer around shallow, horizontal, unsaturated GHX loops, thus simplifying high computational effort needed for full three-dimensional modeling of shallow GHX systems.