Study on the heat transfer performance and reservoir thermal recovery characteristics of medium-deep coaxial heat exchangers during short intermittent operation in cold regions

Based on field test and laboratory experiment, this study established a medium-deep coaxial heat transfer model considering inhomogeneous formation. It examined the impacts of run-stop ratio and intermittent frequency on heat exchange efficiency, revealed the complete evolution process of formation temperature and thermal influence radius during the recovery period, and explored the factors affecting the thermal influence radius. The results indicate that a larger run-stop ratio leads to lower outlet temperature. Increasing the intermittent frequency stimulates the distant reservoir, extends the thermal influence radius and enhances heat extraction, thus improving outlet temperature. When the run-stop ratio is fixed, increasing the daily intermittent frequency from 1 to 4 times raised the average outlet temperature by 0.89 %, 1.73 %, and 2.10 % after 30 days. The minimum outlet temperature increased by 1.715 °C, 2.270 °C, and 2.943 °C, while the thermal influence radius extended by 0.044 m, 0.074 m, and 0.099 m, respectively. During the recovery period, the temperature recovery of the reservoir exhibits temporal and spatial hysteresis. The thermal influence radius is positively correlated with thermal conductivity and negatively with specific heat and density. A larger geothermal gradient and smaller porosity increase the radius, while inlet temperature and flow rate have less impact.