Models of thermal response tests on deep coaxial borehole heat exchangers through multiple ground layers

Highlights: • Numerical simulations of DTRTs with multiple ground layers and geothermal gradient. • Layer-factor model identifies weighting factors on layer thermal conductivity. • 1D radial TRT models are sensitive to sequence of layer thermal conductivities. • Effective ground thermal conductivity depends on heat injection versus extraction. • Effective ground thermal conductivity within ±17% of arithmetic mean of layer values. Ground source heat pumps are often coupled with vertical boreholes in order to heat and cool buildings. As the maximum depth of boreholes has increased, the ranges of undisturbed ground temperature and ground thermal conductivity tend to be wider and more important to consider. These larger variations may affect the analysis of a thermal response test (TRT), which is performed to estimate the effective ground thermal conductivity and borehole resistance. To study potential issues, numerical simulations of distributed thermal response tests have been performed for deep coaxial boreholes penetrating multiple ground layers. The simulated results then serve as data sets for conventional 1D models to analyze TRTs. A layer-factor method has been developed to identify weighting factors on individual-layer properties. These weighting factors show how conventional 1D models determine the effective ground thermal conductivity, which changes with heat injection versus heat extraction, placement of the fluid inlet, and the direction of increasing ground thermal conductivity. For the cases simulated, effective ground thermal conductivities are within ±17% of the arithmetic average of the layer thermal conductivities.