Quantifying the effect of single fractures on the thermal performance of borehole heat exchangers

Ground-source heat pump schemes for space conditioning and thermal storage increasingly use vertical borehole heat exchangers (VBHEs). Current models for VBHE performance often assume homogeneous ground conditions and the absence of flowing groundwater. However, in reality, the ground, especially bedrock, is commonly fractured. In this paper, numerical analysis is used to investigate a range of scenarios in which an open fracture with flowing groundwater may influence the thermal performance of a VBHE. It is intuitive to think that flowing groundwater always increases the heat transfer to and from a VBHE and hence improves their efficiency by reducing the temperature at the borehole wall. However, the results presented in this paper demonstrate that the presence of a fracture within permeable strata can also reduce the performance of a VBHE by locally lowering groundwater flow velocities in the rock matrix. Therefore the influence of a nearby open fracture may either improve or worsen the VBHE thermal performance depending on the fracture orientation relative to the borehole, the regional hydraulic gradient and the ratio of hydraulic permeabilities of the matrix and the fracture.