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. 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 flatpanel, positioned horizontally and edgeways in a shallow trench. Its energetic performance compares favourably with other advanced shapes. In order to design and verify geothermal systems, it 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. We show that the variability associated with the climate parameters plays the most relevant role. It impacts the length of the exchanger for fixed specific power required at the flatpanel, thus affecting the overall design of the geothermal system.