Different Approaches for Evaluation and Modeling of the Effective Thermal Resistance of Groundwater-Filled Boreholes

Abstract

Groundwater-filled boreholes are a common solution in Scandinavian installations of ground source heat pumps (GSHP) due to the particular hydro-geological conditions with existing bedrock, and groundwater levels close to the surface. Different studies have highlighted the advantage of water-filled boreholes compared with their grouted counterparts since the natural convection of water within the borehole tends to decrease the effective thermal resistance Rb*. In this study, several methods are proposed for the evaluation and modeling of the effective thermal resistance of groundwater-filled boreholes. They are based on distributed temperature sensing (DTS) measurements of six representative boreholes within the irregular 74-single-U 300 m-deep borehole field of Aalto New Campus Complex (ANCC). These methods are compared with the recently developed correlations for groundwater-filled boreholes, which are implemented within the python-based simulation toolbox Pygfunction. The results from the enhanced Pygfunction simulation with daily update of Rb* show very good agreement with the measured mean fluid temperature of the first 39 months of system operation (March 2018–May 2021). It is observed that in real operation the effective thermal resistance Rb* can vary significantly, and therefore it is concluded that the update of Rb* is crucial for a reliable long-term simulation of groundwater-filled boreholes.

Experimental evaluation of IDA ICE and COMSOL models for an asymmetric borehole thermal energy storage field in Nordic climate

“…In the IDA ICE model, the effective borehole thermal resistance was set as two constant values for heating and cooling seasons, respectively. However, during the BTES operation, the borehole thermal resistance is changing over time affected by the brine mass flow rate in the borehole heat exchanger which varies according to the heating and cooling demand of the building [60]. In some periods, especially in cooling seasons, the brine mass flow rate in a single borehole heat exchanger was quite low and could be in the laminar flow region.…”

Investigation on the thermal response of steel pipe energy piles with different backfill materials

“…Analytical methods such as the ILSM, SCSM and pile G-Functions are not appropriated for this case, resulting in higher values for the soil thermal conductivity. Previous studies suggested that natural convection increases the heat transfer rate in the annulus region and consequently reduces the effective thermal resistance of the borehole [55,59,60,61]. Additionally, field measurements demonstrated that the groundwater filled borehole resistance is affected by the heat transfer rate and the temperature in annulus region [59,60].…”