A novel model and heat extraction capacity of mid-deep buried U-bend pipe ground heat exchangers

[Display omitted] This study presents a novel simple model that could quickly simulate the heat transfer of mid-deep buried U-bend pipe ground heat exchangers (MBUGHEs). This model simplified a MBUGHE into three independent heat transfer sections: the descending, horizontal, and ascending sections. It was assumed that there was no thermal interference among the different sections because the horizontal pipe was usually very deep and long enough, which meant that each section could be solved separately in independent cylindrical coordinates. CFD software and experimental data were used to validate the established model. The results showed that the model calculation results had high reliability. The model was used to analyze the MBUGHE heat extraction characteristics. The results indicated that the heat extraction capacity increased with the increase of the ground thermal conductivity, ground volumetric heat capacity, buried pipe depth, and horizontal pipe length. The results also showed that the MBUGHE heat extraction capacity had a significant decrease in the first several years, but the decrease was gradually reduced with the increase of the operation time and the decrease became approximately stable after 15 years.