Investigation on local geothermal energy attenuation after long-term operation of ground heat exchanger with considering aquifer effect

The long-term operation of deep buried ground heat exchangers (GHE) for heat extraction would attenuate the local geothermal field. The variation of geothermal energy in the affected area over time is a subject worthy of dedicated study. In this paper, the heat transfer between GHE and surrounding geological strata including aquifuge and aquifer layers is modeled and numerically solved to investigate the variations of GHE performance and attenuation behaviors of ground temperature field. The results show that the reduction ratios of GHE performance in the aquifer and aquifuge layers between 1st and 8th years are 1.3% and 8.9%, respectively, due to the convective heat transfer induced by groundwater in the aquifer layer. The analysis of local temperature variation implies that the groundwater flow leads to the circumferential asymmetry of ground temperature reduction in aquifer layer, which also affects the temperature distribution in geological layers adjacent to the aquifer. The worst ground temperature recovery point appears on the borehole wall in the aquifuge layer, while it appears at a fixed position downstream in the aquifer layer. Taking the ground transient and initial temperature difference greater than 0.5°C as the threshold to characterize the attenuation region, the geothermal field attenuation area of the eighth year in the aquifer is about 4.5 times that of the first year, while it is about 5.1 ∼ 5.5 times that of the first year in the aquifuge layers. The analysis of variations of GHE performance and ground temperature field quantitatively evaluates geothermal attenuation behaviors in aquifer and aquifuge layers subject to the long-term operation of GHEs.