Abstract
The performance of ground-coupled heat pump systems (GCHPs) operating under significant groundwater flow can be difficult to predict due to advective heat transfer in the subsurface. This is the case of the Carignan-Salières elementary school located on the south shore of the St. Lawrence River near Montréal, Canada. The building is heated and cooled with a GCHP system including 31 boreholes subject to varying groundwater flow conditions due to the proximity of an active quarry being irregularly dewatered. A study with the objective of predicting the borehole temperatures in order to anticipate potential operational problems was conducted, which provided an opportunity to evaluate the impact of groundwater flow. For this purpose, a numerical model was calibrated using a full-scale heat injection test and then run under different scenarios for a period of twenty years. The heat exchange capacity of the GCHP system is clearly enhanced by advection when the Darcy flux changes from 6 × 10−8 m s−1 (no dewatering) to 8 × 10−7 m s−1 (high dewatering). This study further suggests that even the lowest groundwater flow condition can be beneficial to avoid a progressive cooling of the subsurface due to the unbalanced building loads, which can have important impacts for design of new systems.
Highlights
Groundwater flow can have a significant impact on the long-term performance of vertical ground heat exchangers (GHEs), especially when the Darcy flux is greater than 1 × 10−7 m s−1 [1,2]
Water is pumped in the active quarry to maintain the groundwater level below the excavation, which affects the local irregularly in the active quarry to maintain the groundwater level below the excavation, which affects groundwater flow regime
Previous studies that involved the numerical evaluation of GHE performance under the influence of groundwater flow identified the conditions where advection becomes the dominant heat transfer mechanism affecting GHE operation, which has been compared in terms of Darcy flux in the following discussion
Summary
Groundwater flow can have a significant impact on the long-term performance of vertical ground heat exchangers (GHEs), especially when the Darcy flux is greater than 1 × 10−7 m s−1 [1,2]. Numerical tools are available to optimize the operation of a GCHP system under the influence of groundwater flow [5,6], an estimate of the specific groundwater flux affecting a GHE field can be difficult to define accurately. Different authors have performed thermal response tests (TRTs) on a single GHE subject to groundwater flow to evaluate the equivalent subsurface thermal conductivity impacted by advection [7,8]. Practitioners commonly using such a heat conduction approach to simulate the long-term operating temperature of GHEs, based on an equivalent subsurface thermal conductivity assumption, have tried to cope with systems influenced by groundwater flow
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