Coal Geology & Exploration


Underground coaxial heat exchanger(UCHE) for medium-deep geothermal has the characteristic of the high heat transfer capacity and outlet water temperature, so it has attracted great attention in the heating industry. However, the current research mainly focus on the heat transfer performance of the UCHE during a single heating period, with insufficient analysis on the long-term heat transfer performance. In this paper, considering the operation characteristics of heat supply in heating season and stopping working in non-heating season for underground coaxial heat exchanger for medium-deep geothermal, a numerical heat transfer model is proposed based on the energy conservation equation. The numerical model is solved by the finite volume method and calculated in Matlab platform, which is used to analyze the heat transfer performance of UCHE during the long-term operation. The results show that the heat transfer performance of the heat exchanger declines with the operating years and the degree of decline decreases year by year, and finally reaches the quasi-steady state. Among them, the decrease proportion of the average heat transfer in the next year is the largest, and the greater the buried depth of the heat exchanger, the smaller the decrease proportion of the heat exchanger. The decline ratios of HTC in the second year are 4.00%, 3.78% and 3.56% for the heat exchangers with lengths of 2 000, 2 500, and 3 000 m, respectively, and the corresponding decline ratios of 30-year operations are 13.7%, 13.1% and 12.4%, respectively. Rock-soil temperature declines and its disturbed radius increase with operating years. During 30-year operations, the disturbed radius increases from 13 m to 105 m for heat exchanger with 2 000 m depth. Furthermore, the disturbed radius is larger for the deeper heat exchanger. The research results demonstrate the evolution of the heat transfer performance of UCHE during the long-term operation, which has the guiding significances to the UCHE design.


geothermal wells, medium-deep, coaxial, heat exchanger, long-term heat transfer




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