New analytical solution for sizing vertical borehole ground heat exchangers in environments with significant groundwater flow: Parameter estimation from thermal response test data

Abstract

Accurate prediction of transient subsurface heat transfer is important in sizing ground heat exchangers in ground coupled heat pump systems. This article examines three analytical solutions for the heat transfer characteristics around closed-loop borehole heat exchangers in significant groundwater flow. The first solution is the so-called moving line source solution, the second is based on the groundwater g-function, and the third is a mass transport solution, adapted here using a mass–heat transport analogy. The ground thermal conductivity, groundwater velocity, and borehole thermal resistance are estimated using a parameter estimation technique in conjunction with the analytical solutions and thermal response test data from two sites in close proximity, one with significant groundwater flow, and the other without. The main difference between the mass–heat transport analogy and the moving line source and groundwater g-function solutions is that the mass–heat transport analogy can account for the effects of thermal dispersion.The mass–heat transport analogy yields a favorable comparison to field test data with a very high groundwater flow rate, while the other solutions do not produce a realistic comparison, implying that thermal dispersion is an important parameter in subsurface heat transfer, at least in situations with relatively high groundwater flow rates.

Acknowledgments

Andrew Chiasson, PhD, PE, PEng, Member ASHRAE, is Assistant Professor. Amanda O’Connell, LEED AP BD + C, Associate Member ASHRAE, is Mechanical Engineer.