Long-Term Temperature Evaluation of a Ground-Coupled Heat Pump System Subject to Groundwater Flow

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.

Case Studies of Geothermal System Response to Perturbations in Groundwater Flow and Thermal Regimes

“…The conditions that best match the observation point (h = 20.7 m a.s.l.) are a hydraulic conductivity of 1.26 × 10 −5 m/s and a net recharge of 100 mm/year (Jaziri et al 2016), both in agreement with the available regional groundwater flow assessment (Carrier et al 2013).…”

“…This study provides further evidence that even the lowest groundwater flow conditions expected at the site can be beneficial to avoid a progressive cooling of the underground over the expected life of the system due to the unbalanced heating and cooling loads. In a previous study, (Jaziri et al 2016) simulated the operation of the GCHP system with a heat conduction approach considering an equivalent subsurface thermal conductivity up to 3 W/m/K and assumed affected by groundwater flow. The BHE operating temperatures at the beginning of the simulations were similar to those obtained with FEFLOW and presented in this paper for Scenario 1 (low groundwater flow), but decreased by 4 to 6 • C over the 20 years of system operation.…”

“…(a) Location of the study site with hydraulic boundary conditions (h1 and h2); (b) conceptual geological model; (c) 3D numerical model showing the boundary conditions and initial temperature for each layer (redrawn from Jaziri et al 2020). …”

“…Initial site characterization included two TRTs, carried out before and after the BHE field installation, which revealed a bulk subsurface thermal conductivity of 2.58 W/m/K and 2.27 W/m/K, respectively (Jaziri et al 2020 and references therein). Thermal conductivity values are in agreement with literature data (Bédard et al 2017; Raymond et al 2017; Raymond et al 2019); and the difference between the two tests is assumed to be due to changes in the groundwater flow regime near the school.…”

“…Thermal conductivity values are in agreement with literature data (Bédard et al 2017; Raymond et al 2017; Raymond et al 2019); and the difference between the two tests is assumed to be due to changes in the groundwater flow regime near the school. Rock samples of the gabbro dykes, shales, and calcarenite, collected from the quarries, showed an average thermal conductivity of 1.85, 2.64, and 3.5 W/m/K, when respectively measured in the laboratory (Jaziri et al 2016). Hydraulic conductivity and recharge were assessed by reproducing the hydraulic head measured in the abandoned quarry (h) considered as an observation point and using an analytical solution for steady‐state flow in an unconfined aquifer (Fetter 2001).…”