Thermal response test analysis for U-pipe vertical borehole heat exchangers under groundwater flow conditions

Conventional models used in the analysis of thermal response test data only consider conduction as heat transfer mechanism. In cases where presence of groundwater is detected, convection heat transmission plays an important role, so its influence must be determined in the calculation of the effective thermal conductivity, usually overestimated in these situations, increasing its value the higher the power injected and the time elapsed. In this work, based on the data collected in a borehole located at UPV (Valencia) in which have been carried out three thermal response tests with different characteristics, has been implemented a variation of the finite line source model introducing an expression for the effective thermal conductivity formed by two terms, one static unaffected by underground flow and another dynamic that depends on time. Analyzing the data in the model developed and in the finite line source and infinite line source models, the results show that the new model estimates accurately the conductivity value unaffected by underground flow regardless the power injected or the time elapsed in the test, with differences between the results obtained in the analysed tests and average thermal conductivity of 1,4%, compared to the conventional models in which this difference is 27%. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

Conventional models overestimate the effective thermal conductivity in the presence of groundwater during thermal response tests. A new model, which accurately estimates the thermal conductivity unaffected by underground flow, was developed and tested in borehole experiments, showing a significant improvement in accuracy compared to traditional models.