3D Numerical Modelling and Sensitivity Analysis of a BTES System coupled with Road Thermal Collector.

The decarbonization of the space heating sector, which accounts for approximately 35 % of greenhouse gas emissions within the European Union, represents a critical priority for achieving climate neutrality by 2050. Fifth-generation district heating and cooling (5GDHC) networks offer an unprecedented technological framework, enabling the integration of low-temperature industrial waste heat and urban solar energy, which can be harvested via asphalt pavements (Road Thermal Collectors - RTC). However, the seasonal mismatch between summer energy availability and winter heating demand necessitates the implementation of Borehole Thermal Energy Storage (BTES) systems, acting as seasonal thermal batteries in the subsurface. This research thesis details the development and evaluation of a fully coupled, three-dimensional finite element numerical model constructed in FEFLOW to characterize the thermodynamic performance of a compact, concentric BTES field. The layout consists of 24 parallel Double U-tube Borehole Heat Exchangers (BHEs) arranged along three concentric regular octagonal rings, covering a planimetric footprint of approximately 87.58 m². In this framework, the thermal energy input driving the subsurface storage originates directly from the conversion of incident solar radiation into sensible heat within the asphalt pavement, which is harvested via RTC technology. This collected solar thermal surplus is then injected into the BTES system during the summer charging season, effectively coupling surface energy harvesting with seasonal geothermal storage. To identify the dominant physical, geological, and operational drivers of the geothermal reservoir, a systematic multi-parametric sensitivity analysis was executed using the One-Factor-at-a-Time methodology over a two-year operational window. The numerical experiments systematically evaluated the following factors: (I) the influence of subsurface water saturation, (II) the deployment of a superficial extruded polystyrene (XPS) insulation cover, (III) the impact of active groundwater advection within an intermediate sand aquifer, (IV) alternative drilling depths and aspect ratios, (V) the sizing of the solar RTC collector, (VI) varying heat transfer fluid volumetric flow rates, and finally (VII) alternative BHE geometries (Single-U, Double-U, Coaxial Cx-A, and Coaxial Cx-C). The sensitivity results guided the design of a unified, optimized system configuration (fully saturated ground, 0.50-meter top insulation, extended 40-meters borehole active depth, and an optimized laminar flow rate of 4.00 m³/day) subjected to a realistic 6-months active winter heat extraction cycle under hydraulic flow reversal. The optimized BTES demonstrated a good thermodynamic performance, extracting 51.70 GJ of thermal energy from the ground. Symmetrically, the system maintained a perfectly constant fluid temperature lift of 5.00 °C, returning an average supply temperature of 23.65 °C. Furthermore, the analysis deconstructed a powerful "thermal shielding" feedback loop: by actively extracting heat, the average core temperature is lowered, flattening the radial temperature gradient with the undisturbed ground and reducing winter conductive losses by 46.85% compared to the passive winter resting state. Finally, the optimized system achieved a single-cycle Round-Trip Efficiency (RTE) of 38.87 % in only its second operational year. This research validates the capacity of parametric optimization to bypass the typical low-efficiency startup bottleneck of newly commissioned fields, establishing a robust computational and engineering foundation for highly efficient, decentralized seasonal thermal energy storage in urban environment.