Meso-scale investigation of the effects of groundwater seepage on the thermal performance of borehole heat exchangers

In view of the Net Zero Initiative, geothermal energy has been widely used. The borehole heat exchanger is a device that extracts geothermal heat from a borehole. Understanding the thermal performance of it is crucial for the analysis and design of a ground source heat pump system. In this paper, a novel methodology is proposed to evaluate the thermal performance of borehole heat exchangers based on a meso-scale model. With the images showing the meso-structure of the geomaterial, a method to quantify grain ellipticity, grain roughness, porosity and grading is proposed, with which the meso-structure of the geomaterial is reconstructed in a finite-element model. The Monte-Carlo simulation is then used in conjunction with the finite-element method to calculate the thermal release coefficient of borehole heat exchangers probabilistically. After validating the proposed methodology using experimental results, the effects of a list of factors on the thermal performance of borehole heat exchangers are investigated. The results show that grain ellipticity and roughness significantly affect the performance of borehole heat exchangers. The present study contributes to the field by quantifying the interactive effects of groundwater seepage and the physical characteristics of geomaterials and providing recommendations on the design of borehole heat exchangers.