Research on the long-term operation performance of deep borehole heat exchangers array: Thermal attenuation and maximum heat extraction capacity

Abstract

The long-term operation performance and thermal attenuation of deep borehole heat exchangers (DBHEs) array are important for their stable and safe operation. This paper proposed a semi-analytical method to quantitatively evaluate the 100-years operation performance of DBHEs array. Numerical simulation was utilized to determine the heat flux per depth of DBHE, while the finite line heat source model was modified to calculate the ground temperature variation. This allowed to obtain the ground borehole wall temperature, serving as a boundary condition for calculating the water temperature distribution in DBHE using an analytical solution. The results revealed that the line space of DBHEs array directly affected the allowed maximum accumulated heat extraction per DBHE per heating season (Qa,max). With line space decreasing from 100 m to 10 m, the Qa,max decreased from 0.3% to 19.1% compared to the single DBHE. Therefore, a line space larger than 30 m was recommended. Considering practical project with limited space, the inclined DBHEs were recommended in depth direction. For DBHEs array with a line space of 10 m at the surface ground and 100 m at a depth of 2500 m, Qa,max was only 3.2% smaller compared to the single DBHE, which showed an improvement over DBHEs array with a line space of 50 m. The proposed semi-analytical method could be applied in system design, to ensure the long-term safety and stability of DBHEs array.