Determination of groundwater flow direction in thermal response test analysis for geothermal heat pump systems

Abstract

A methodology was proposed to determine the groundwater flow direction from a thermal response test analysis using three boreholes. The methodology was verified using sample test data generated from a three-dimensional numerical model for the borehole ground heat exchangers. It was found that the groundwater flow velocity and the borehole separation had significant effects on the correctness of the estimated groundwater flow direction. A smaller borehole separation and a higher groundwater flow helped improve the traveling speed of the heat front and, consequently, the thermal interference effect from adjacent boreholes. This was crucial for the correct prediction of the groundwater flow direction in the analysis. The adoption of a longer test period also strengthened the borehole thermal interference and, hence, the accuracy of the estimated groundwater flow direction. Besides, the minimum groundwater flow velocity capable of being determined with confidence from the analysis could also be decreased. Finally, the use of an unequally spaced borefield could further enhance the effectiveness of the methodology.