The effect of fractures in aquifers on the performance of borehole ground heat exchangers

Abstract

Ground-source heat pump schemes for space conditioning and thermal storage increasingly use vertical borehole heat exchangers (VBHEs) because they can be installed in a wide range of geological conditions. Current models for VBHE performance often assume homogeneous ground conditions. However, in reality, hydrogeological conditions are rarely simple and homogeneous, and bedrock is commonly fractured. Application of the typical design assumption of homogeneous and isotropic ground conditions can give erroneous results. It is known that for a single fracture in a low hydraulic conductivity matrix, the flow in the fracture increases the apparent thermal conductivity of the ground. Therefore, the presence of a fracture improves the estimated thermal performance of a VBHE and exacerbates downstream thermal impacts. However, studies investigating VBHE performance near to a flowing fracture installed in a matrix with considerable groundwater flow are lacking. This study uses 2D and 3D numerical modelling to investigate a range of possible hydrogeological scenarios in which an open, flowing fracture may influence the long-term thermal performance of a VBHE. The key question considered by this study is: when does an open fracture improve (and when does it worsen) the thermal performance of a VBHE, compared with the thermal performance estimated assuming a homogeneous host rock? To answer this question, the temperature change at the VBHE wall, the mean temperature change of the heat transfer fluid, the extent of the downstream thermal plume, and the time to reach steady state were all used as the indicators of the VBHE performance. For simplicity, the analysis considered 30 years of the continuous VBHE operation under the constant thermal loading. The analysis demonstrated that a fracture could have positive or negative effect on the thermal performance of a VBHE. The outcome depends on the interplay of two fracture effects. Firstly, the ability of a fracture to change the local groundwater velocities in the aquifer matrix (which leads to an increase or decrease in the local apparent thermal conductivity of the matrix). Secondly, the ability of a fracture itself to increase the thermal transport from a VBHE. The fracture can reduce the thermal transport from the VBHE if the first fracture effect is dominant and the VBHE is located in the area where the groundwater velocity has been locally reduced. This will lead to the increase in the estimated temperature change at the borehole compared with the case when the aquifer is assumed to be homogeneous. The overall fracture effect on the VBHE depends on which of the two fracture effects is dominant. The impact of the fracture is the most significant in cases when the groundwater flow is moderate (0.01 - 0.1 m day-1). At the higher groundwater flows, the impact of thermal dispersion in reducing the temperature change at the borehole is greater than the impact of the fracture in most cases. For moderate groundwater flows, significant thermal dispersion will act to exacerbate the adverse fracture impacts and reduce its positive impacts. For slow groundwater flows in an aquifer, a fracture near a VBHE can significantly increase the thermal transport, even when the volumetric flow rate in the fracture is small. Therefore, when groundwater flow in an aquifer is slow, the thermal performance of a VBHE is likely to be beneficially influenced by a fracture. Fractures in bedrock aquifers potentially can have a positive or negative effect on the thermal performance of a VBHE. Therefore, the uncertainty in the long-term thermal performance of VBHEs and their downstream thermal impacts can be reduced when the assumption of homogeneous and isotropic ground conditions is justified.

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Identifiers

ORCID for Oleksandra Pedchenko: orcid.org/0000-0003-4802-7846

ORCID for William Powrie: orcid.org/0000-0002-2271-0826

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