Super-long gravity heat pipe system for simultaneous heating and cooling in geothermal sites with non-linear temperature gradients

Published super-long gravity heat pipe (SLGHP) research assumes geothermal temperature increases linearly with depth. This assumption fails in specific geological settings where temperatures plateau, decoupling borehole depth from higher extraction temperatures. This work presents a numerical analysis of an 1800 m ammonia SLGHP with patented mid-depth vapor extraction, integrated with heating and cooling heat pumps for a residential complex in Slovenia where formation temperature reaches 81 °C regardless of depth. The model was validated against published SLGHP field and simulation data with mean absolute percentage errors of 1.60% and 1.30%, respectively. The patented packer configuration limits heat exchange to the high-temperature zone below the packer, where formation temperature exceeds the working fluid saturation temperature. This achieved 23% higher extraction than conventional designs at T c = 50 °C (185 kW versus 150 kW) and extended the operating range to over T c = 70 °C. The integrated system achieved a heat pump COP of 4.22 during peak winter demand, rising to over 5.75 with waste heat recovery and direct vapor condensation preheat. Over the 120-day heating season, the system achieved a heating SPF of 6.19. During night-time, heating demand was met without the heat pump. During a representative winter week, diurnal cycling allowed extraction temperature recovery from 57 °C to 73 °C overnight, while a thermal buffer recovered 88.4% of cooling waste heat. The results demonstrate SLGHP viability for low-enthalpy plateau-temperature resources and present the first integration of simultaneous heating and cooling with a single deep heat pipe in geologically unfavourable settings.