Interpretation of TRT data in the presence of natural convection and groundwater flow in saturated ground

Abstract

Ground thermal properties play important roles in the design of geothermal heat exchangers (GHEs). Constant ground thermal properties, as often considered in analysis and design of GHEs, often do not account for the coupled effect of thermal and pore fluid flow in saturated ground. This paper presents a series of advanced finite element analyses (FEAs) of thermal response test (TRT) on a geothermal borehole heat exchanger (BHE) installed in uniform and multi-layered saturated soil deposits. Considering coupled thermo-hydro flow in saturated ground, FEAs explore the effects of ground hydraulic characteristics (e.g., groundwater flow, soil permeability), power injection rate, and duration of thermal response test on the value of effective ground thermal conductivity keff. Results show that the value of keff, in the presence of coupled thermo-hydro flow condition, can be multiple times of the thermal conductivity value estimated considering only conductive heat transport in coarse grained soil. Example calculations following two available BHE design standards confirm that consideration of soil permeability and selection of appropriate duration of TRT may lead to significant reduction in the design length of BHEs.