Uncertainty-based analysis of variations in subsurface thermal field due to horizontal flat-panel heat exchangers

Geothermal energy is produced by coupling a heat pump with the ground, resorting to ground heat exchangers (GHEs) that can be installed in vertical or inclined boreholes or horizontally in shallow ground. Horizontal GHEs are easy to be installed and maintained, more compliant with environmental regulations, and generally do not interfere with groundwater systems. These advantages are balanced by a decidedly lower specific power. To overcome this deficiency, the shape of the exchangers plays a relevant role. Here, we consider a new shape devised in the form of a flat-panel, positioned horizontally and edgeways in a shallow trench. Its energetic performance, tested both numerically and experimentally, compares favourably with other advanced shapes. In order to design and verify geothermal systems is crucial to predict accurately the soil thermal field around the exchanger. This prediction is generally compromised by the uncertainty associated with (i) the thermo-physical properties of the soil and (ii) the solar impact on surface energy balance, that mainly controls the thermal energy storage in the first layer of the subsurface environment. In this context, global sensitivity analysis (GSA) may be performed to delineate the most significant sources of uncertainty and address measurements accordingly. Sensitivity studies of other horizontal GHEs have been developed without resorting to GSA. Here, we present an effective approach for the characterization of the uncertainty associated with the variations in the soil thermal field induced by a flat-panel. First, a numerical model describing the transient heat transport problem is developed; then, GSA is efficiently performed via model reduction techniques, in order to minimize the computational cost. We show that the accuracy in the surface energy balance is more substantial than that related to the soil properties. It impacts the length of the exchanger for fixed specific power required at the flat-panel, thus affecting the overall design of the geothermal system