In Situ Experiment and Numerical Model Validation of a Borehole Heat Exchanger in Shallow Hard Crystalline Rock

Abstract

Accurate and fast numerical modelling of the borehole heat exchanger (BHE) is required for simulation of long-term thermal energy storage in rocks using boreholes. The goal of this study was to conduct an in situ experiment to validate the proposed numerical modelling approach. In the experiment, hot water was circulated for 21 days through a single U-tube BHE installed in an underground research tunnel located at a shallow depth in crystalline rock. The results of the simulations using the proposed model were validated against the measurements. The numerical model simulated the BHE's behaviour accurately and compared well with two other modelling approaches from the literature. The model is capable of replicating the complex geometrical arrangement of the BHE and is considered to be more appropriate for simulations of BHE systems with complex geometries. The results of the sensitivity analysis of the proposed model have shown that low thermal conductivity, high density, and high heat capacity of rock are essential for maximising the storage efficiency of a borehole thermal energy storage system. Other characteristics of BHEs, such as a high thermal conductivity of the grout, a large radius of the pipe, and a large distance between the pipes, are also preferred for maximising efficiency.

The Effect of Shank-Space on the Thermal Performance of Shallow Vertical U-Tube Ground Heat Exchangers

“…The effects of the attributes mentioned above were investigated numerically by means of computational fluid dynamics (CFD) simulations. Whereas in situ experiments, referred to as thermal response tests [11], are frequently used to characterize the performance of ground heat exchangers, given the advancement of numerical modeling, it has now become customary to use numerical modeling, either to complement these in situ analysis [37] or else to extend validated models to research new aspects. Numerical modeling was carried out in ANSYS FLUENT 16.2 [38], which was used to perform three-dimensional (3D) steady-state CFD calculations in order to simulate the heat transfer process between the fluid and the grout/ground combination and the resulting temperature distributions for the various ground heat exchanger configurations.…”

Numerical modeling and validation of a large-scale borehole thermal energy storage system in Finland

“…The density and specific heat capacity of the groundwater were estimated to be 999 kg/m 3 and 4200 J/kg•K, respectively. The effective thermal conductivity, specific heat capacity and density of the ground were 3.3 W/(mK) (Todorov, Vallin, et al, 2021) , 725 J/(kgK) (Janiszewski et al, 2018) and 2500 kg/m 3 (Janiszewski et al, 2018). The undisturbed ground temperature and the geothermal temperature gradient were 8.7 ℃ and 0.0119 ℃/m, obtained from on-site thermal response test.…”

Numerical Study on the Thermal Performance of a Single U-Tube Borehole Heat Exchanger Using Nano-Enhanced Phase Change Materials

“…In a thermal response test (TRT) conducted by Sapinska-Sliwa et al [7] for three types of BHEs, for instance, single U-tube, double U-tube, and coaxial BHE, the last configuration was found to be the best one in achieving the effective thermal conductivity of the ground. In another study, carried out experimentally and numerically by Janiszewski et al [8], a single U-tube BHE was used to inject thermal energy into the surroundings. It was proven that a higher thermal conductivity of the pipe and backfill as well as a larger pipe distancing and pipe radius are essential for improving the borehole thermal energy storage (BTES) system efficiency.…”