Thermal attenuation and heat supplementary analysis of medium-deep coaxial borehole system-based on a practical project

The medium-deep downhole coaxial heat exchanger has been widely investigated in terms of its clean, envi-ronmentally friendly, stable and reliable characteristics. In this work, the heat transfer performance including outlet water temperature, daily heating capacity, daily heat consumption and coefficient of performance of ground source heat pump system are analyzed on the basis of practical project data for one and consecutive heating period. Results show that when medium-deep downhole coaxial heat exchanger is operated for consecutive years, the outlet water temperature is affected by the heating demand, and heating capacity de-creases significantly because of the cooling capacity accumulated in surrounding rocks during non-heating period, which cannot be sufficiently used during heating period. With attenuation analysis, the heat capacity decreases in a negative change with attenuation rate of 17.32%, 16.17%, and 8.82% respectively from 2nd to 4th heating period. And the preventive measures should be taken to avoid heat capacity from decreasing during operation period. Three solar supplemental heat schemes are proposed and can alleviate the heat attenuation over time. The results show that continuous heat supplement in non-heating period of each year is an optimal scheme with neglecting the little cost difference of solar collector.