New guidelines for the application of the infinite line source method for thermal response tests on atypical borehole heat exchanger configurations

Abstract

This study explores the application of the infinite line source when used to evaluate the temperature response of a thermal response test on a series of boreholes. This can occur in practical applications in which existing borehole fields need recharacterization when: field operation has persisted over multiple years, field modifications have been made, or enhanced understanding of the thermal properties are required for control strategies. In this study new insights towards the start time and duration of the thermal response test indicate that at a minimum duration of 75 h, the data for evaluation should begin after the first 50 h. The effect of horizontal piping connected to a borehole is also evaluated and characterised with respect to the effective length of the borehole heat exchanger. Next, thermal response tests performed on boreholes connected in series are evaluated and it was found that for a Fourier number less than 0.06 the infinite line source is applicable. Finally, the results from the case study are then applied to the model of experimental TRTs performed on boreholes in series with horizontal header pipes. The model uses the experimental inlet mass flow rate and temperature to investigate the heat transfer and thermal interaction between the individual boreholes to capture the outlet temperature. Thermal interaction along the depth of the boreholes are shown to have the majority of the impact on the ILS output. The ILS is able to estimate the thermal conductivity within 5% of the input value for these unique borehole arrangements.