Performance Analysis of Different Borehole Heat Exchanger Configurations: A Case Study in NW Italy

Highlights: What are the main findings? A good knowledge of the underground parameters and of the aquifer characteristics are essential in the design of borehole heat exchangers as part of a shallow geothermal plant integrated in a district heating and cooling grid. Borehole heat exchangers with a coaxial configuration outperform double U pipes in terms of energy efficiency, especially during intermittent operation modes of the geothermal heating system. What is the implication of the main finding? Proper design of a shallow geothermal plant can potentially reduce the cost of drilling boreholes and make the installation easier on site, also improving the sustainability of urban environments. A geothermal-based district heating and cooling grid can be combined with other renewable energies, achieving the best thermal energy performance, improving smart energy systems, and the decarbonisation of the building sector. The central role of heating and cooling in energy transition has been recognised in recent years, especially with geopolitical developments since February 2022 which demand an acceleration in deploying local energy sources to increase the resilience of the energy sector. Geothermal energy is a promising and vital option to optimize heating and cooling systems, promoting sustainability of urban environments. To this end, a proper design is of paramount importance to guarantee the energy performance of the whole system. This work deals with the optimization of the technical and geometrical characteristics of borehole heat exchangers (BHEs) as part of a shallow geothermal plant that is assumed to be integrated in an already operating gas-fired DH grid. Thermal performances of three different configurations were analysed according to the geological information that revealed an aquifer at −36 m overlying a poorly permeable marly succession. Numerical simulations validated the geological, hydrogeological, and thermo-physical models by back-analysing the experimental results of a thermal response test (TRT) on a pilot 150 m deep BHE. Five-year simulations were then performed to compare 150 m and 36 m polyethylene 2U, and 36 m steel coaxial BHEs. The coaxial configuration shows the best performance both in terms of specific power (74.51 W/m) and borehole thermal resistance (0.02 mK/W). Outcomes of the study confirm that coupling the best geological and technical parameters ensure the best energy performance and economic sustainability.