A Study on Optimizing the Thermal Performance of Coaxial Heat Exchanger Systems in Medium-Deep Geothermal Wells

Abstract: Medium-deep geothermal energy is a key renewable source, but existing coaxial downhole heat exchanger (DHE) systems suffer from low efficiency and temperature decay. This study evaluates the impacts of circulating flow rate, inlet temperature, and operation mode on DHE performance, using field data from two geothermal wells (Well A: 3200m, 130.5$^{\circ}$C; Well B: 2500m, 103.3$^{\circ}$C). Results show that under an optimal condition (LC3: 50 m$^3$/h, 30$^{\circ}$C), Well A's heat extraction rate increased from 35% to 42%, with its outlet temperature rising from 15$^{\circ}$C to 20$^{\circ}$C. In contrast, Well B's rate decreased from 15% to 5%. After one week of continuous operation, the outlet temperature of Well A dropped from 55.7$^{\circ}$C to 16.5$^{\circ}$C, and Well B's from 68$^{\circ}$C to 17$^{\circ}$C. Adopting an intermittent mode (16h operation, 8h shutdown daily) reduced the temperature decay rate by approximately 10%. Based on these findings, we propose optimization strategies: controlling flow rate to 35m$^3$/h, maintaining an inlet temperature of 6--10$^{\circ}$C, and implementing intermittent scheduling. This work provides guidance for the efficient design and sustainable operation of DHE systems.