Comparative analysis of Different Closed-loop Borehole Heat Exchanger Thermal Performance in shallow High-Temperature Environment

Abstract Closed-loop geothermal systems offer a promising pathway for exploiting high geothermal gradients in volcanic environments while avoiding the operational and environmental risks associated with open-loop circulation. However, the influence of borehole trajectory and geometric configuration on the long-term thermal performance of such systems remains insufficiently constrained, particularly under shallow, high-temperature volcanic conditions. This study presents a systematic numerical comparison of five closed-loop borehole heat exchanger (CBHE) configurations: i) vertical coaxial (V-CBHE), ii) deviated coaxial (D-CBHE), iii) horizontal coaxial (H-CBHE), iv) U-loop (U-CBHE), and v) single-lateral (SL-CBHE), using geological and thermal conditions representative of Vulcano Island (Italy). A fully coupled 1D wellbore–3D formation thermo-hydraulic model was developed in a FEM commercial software to simulate conductive heat transfer and fluid circulation over a 25-year operational period. Sensitivity analyses were conducted to evaluate the effects of borehole geometry, inclination depth and angle, horizontal extension length, injection temperature, flow rate, and insulation strategies. Results demonstrate that borehole geometry exerts a dominant control on long-term performance, surpassing the influence of operational parameters. Lateral configurations significantly outperform vertical and deviated systems by extending heat-exchange length within high-temperature formations, increasing fluid residence time, and distributing thermal drawdown over a larger rock volume. After 25 years of continuous operation, horizontal, U-loop, and single-lateral systems stabilise at production temperatures of approximately 147–165°C, compared to 96–101°C for vertical and deviated configurations. Inner-pipe insulation effectively mitigates internal thermal short-circuiting, particularly during early operation. Although electrical power estimates represent idealised upper bounds, the relative performance ranking among configurations remains robust. These findings highlight trajectory design as a critical lever for optimising closed-loop geothermal systems in shallow, high-enthalpy volcanic environments.