Overcoming uncertainties associated with local thermal response functions in vertical ground heat exchangers

Abstract

The short-term performance of ground heat exchangers (GHEs) is crucial for the optimal design of ground-source heat pumps (GSHPs), enhancing their contribution to sustainable energy solutions. Local short-time thermal response functions, or shorttime g-functions (STGFs) derived from thermal response tests (TRTs), are of great interest for predicting the heat exchange due to their fast and simple applicability. The aim of this work is to perform a sensitivity analysis to assess the impact of thermal parameter variability and TRT operating conditions on the accuracy of the average fluid temperature (Tf ) predictions obtained through a local STGF. First, the uncertainties associated with the borehole thermal resistance (Rb), transmitted from the soil volumetric heat capacity (CS) or some models dependent on GHE characteristics, such as the Zeng model, were found to have a low impact in Tf resulting in long-term deviations of ±0.2 K. Second, several TRTs were carried out on the same borehole, changing input parameters such as the volumetric flow rate and heat injection rate, in order to obtain their corresponding STGF. Validation results showed that each Tf profile consistently aligned well with experimental data when applying intermittent heat rate pulses (being the most unfavorable scenario), implying deviations of ±0.2 K, despite the variabilities in soil conductivity (λS), soil volumetric heat capacity (CS), and borehole thermal resistance (Rb).