Evaluation of a Closed-Loop coaxial borehole heat exchanger in High-Temperature volcanic environment

This study evaluates the technical feasibility and thermal performance of a closed-loop coaxial borehole heat exchanger (CBHE) for geothermal energy extraction in high-temperature volcanic environments, with application to Vulcano Island, Italy. A two-dimensional axisymmetric finite-element model was developed in COMSOL Multiphysics to simulate coupled heat transfer and fluid flow within a multi-layered geological formation using temperature- and pressure-dependent water properties. Model validation against existing deep borehole field data demonstrated good agreement, with mean absolute percentage error below 5%. Parametric analyses assessed the effects of injection temperature (80–90 ◦C), flow rate (3–15 l/s), borehole depth (400–1000 m), and insulation configuration on system performance. Results indicate that injection temperature and flow rate showed limited influence on long-term electric power output. Increasing borehole depth significantly enhances heat extraction, with a 1000 m insulated CBHE achieving up to 11 MWe of potential electric power and outlet temperatures of 106 ◦C after long-term operation. At shallow depths, combined annulus and inner pipe insulation increased performance by 7.8 %, whereas at 1000 m depth, inner pipe insulation alone improved heat recovery by over 10 %. The findings demonstrate that optimized CBHE configurations can provide stable and sustainable thermal energy recovery in high-enthalpy volcanic systems, offering a technically feasible and environmentally safe alternative to conventional hydrothermal and enhanced geothermal systems. This research also contributes to the development of closed-loop geothermal technologies for clean energy generation in similar high-enthalpy regions and decarbonization of volcanic island regions.