Migration Experiment of the Leaked Pollutants in Borehole Heat Exchangers

Abstract

With the large-scale application of ground source heat pumps, the risk of potential groundwater pollution caused by the expiration of the life of the buried pipe and the leakage of the working fluid is also increasing, however, no relevant research has been found yet. Based on the actual working conditions of borehole heat exchangers (BHEs), an experimental system for the migration of contaminants was established. The migration of leaked contaminants of single pipe under continuous heating, continuous cooling and periodic temperature boundary conditions were carried out. The results show that: (1) During an operation period of the borehole heat exchanger, the concentration of contaminants increases with the increase of fluid temperature inside the tube, reaches the maximum at the end of the heat injection process; then gradually decreases; the minimum reaches at the end of the heat extraction process. Moreover, the further away from the heat source, the later the concentration peak appears. (2) The natural convection is generated by the temperature difference between the tube wall and porous media during heat injection, which drives the contaminants to diffuse upward; however, there is no obvious change during heat extraction. (3) No obvious contaminants were detected in the radial direction of the porous media under periodic temperature boundary conditions, this is because the density of the contaminants is larger, which are more affected by gravity during migration and less radial diffusion.