Estimating groundwater heat exchange in a standing-column well by injection of a bromide tracer

Abstract

Accurate quantification of subsurface advective and conductive heat transport is necessary in the design length determination of borehole heat exchangers as used with geothermal heat pump systems. This article describes the field measurement and quantification of the effects of advective groundwater flow on the thermal performance of a standing column well (SCW) borehole heat exchanger. Advective groundwater flow is rarely, if ever, quantified by geothermal heat pump system installers and practitioners, and it is commonly misconceived that if large volumes of groundwater are encountered during drilling of an SCW (or other types of borehole heat exchangers for that matter) that advective flows of groundwater are also high, thus allowing reduction in design borehole lengths as compared to cases with purely conductive heat transport alone. In the present research, the advective heat exchange potential of groundwater in a standing-column installation was investigated by introducing and circulating a bromide tracer solution, and monitoring its concentration over five days. The test was performed on a 457-m (1500-ft) deep standing-column ground-source heat pump well that produced a significant flow (approximately 18.9 Ls−1 (300 gpm)) during drilling. Bromide concentrations declined over time by approximately 50% while the well was circulated at a rate of roughly 4.4 Ls−1 (70 gpm) with no bleed discharge (the groundwater returned to the well equaled the flow extracted from the well). Based on the volume of standing water in the well, the estimated dilution by fresh groundwater is approximately 0.0126 Ls−1 (0.2 gpm), or less than 0.3% of the circulation rate. The source of this water is attributed to advective flow through the fractured-bedrock aquifer. Because the groundwater exchange is a small fraction of the circulation rate, the results indicate that the flow encountered during drilling is a poor measure of the potential for groundwater exchange in a circulating standing-column well (without bleed discharge). Results of this work indicate that heat transport in the subsurface due to advective groundwater flow is a negligible heat transfer process relative to conduction in the geologic environment examined in this study.