Spatiotemporal characterization of a ground heat exchanger (GHE) operating in partially saturated clay formation

One of the key challenges in shallow geothermal technology is improving the thermal performance of ground heat exchangers (GHEs) while reducing operational costs. Existing enhancement techniques are typically applied uniformly along the GHE depth, despite field data showing non-uniform thermal performance. This paper presents a spatiotemporal characterization of a single u-pipe GHE, consolidating theoretical framework, laboratory characterization, and numerical modeling to identify where and when coupled heat and moisture transfer becomes significant within the GHE geometry. The coupled model is validated against a laboratory column test under ambient, and non-insulated boundary conditions and applied to characterize the three-dimensional axial and radial extent of soil drying and wetting around a GHE operating in heat rejection mode. Results reveal an asymmetric temperature and moisture distribution, with greater desaturation near the inlet leg, concentrated in the upper 40–50% of the GHE depth and extending 1.21 m - 1.52 m (4 - 5 ft) radially from the pipe. This desaturation is self-limiting, stabilizing within one to two months of operation, and corresponds to an approximately fivefold increase in matric suction near the pipe, from approximately 348 kPa (7268.13 psf) to approximately 1851 kPa (38,658.94 psf) over four months. The associated reduction in thermal conductivity and heat capacity substantially alters the heat exchange capacity of the surrounding soil. These findings establish the spatiotemporal basis for future design and operational strategies, such as targeted use of high-conductivity materials near the GHE inlet and periodic flow reversal, to improve the cost-effectiveness of shallow geothermal systems.