A numerical study of the short- and long-term heat transfer phenomena of borehole heat exchangers

Abstract

This thesis contributes an in-depth comparative study of u-tube and coaxial borehole heat exchangers. While it is widely accepted that the lower resistance of the coaxial heat exchanger should result in a performance advantage, the findings of several studies comparing the heat exchanger configurations did not definitively establish the mechanisms causing differences in performance. This study employs numerical modelling to consider heat exchangers over a broad range of time scales and under carefully controlled geometry and flow conditions, resulting in the identification of the key parameters influencing borehole heat exchanger performance.

The first part of this study consists of a comparison of u-tube and coaxial heat exchangers under continuous loading. A detailed conjugate heat transfer numerical model was developed in OpenFOAM, designed to capture both short and long time scales of heat exchange, necessary to understand the nuanced differences between designs. A novel transient resistance analysis was employed to understand the dominant factors influencing performance. This study established that marginal differences exist between u-tube and coaxial borehole heat exchangers (BHEs) when operated continuously long term but that greater differences occur early in operation.

The second phase of this investigation provided a framework for analysing borehole heat exchanger performance during intermittent operation, while also comparing u-tube and coaxial designs. During this study, it was found that reducing operating time, improving the the rate of the ground's recovery to its original temperature, and lowering the duty cycle improved BHE performance. Transit time was identified as a influential time scale, below which heating at the outlet was limited. Further, the benefits of operating below the transit time were mitigated by design-specific interaction between inlet and outlet flows. Finally, this study found that non-dimensionalizing operating time by transit time causes the differences between u-tube and coaxial performance to vanish, leading to the conclusion that differences in BHE performance are caused by variations in flow rather than thermal mass.