On the Influence of Operational and Control Parameters in Thermal Response Testing of Borehole Heat Exchangers

Abstract

Thermal response test (TRT) is a common procedure for characterization of ground and borehole thermal properties needed for the design of a shallow geothermal heat pump system. In order to investigate and to develop more accurate and robust procedures for TRT control, modelling, and evaluation in semi-permeable soils with large water content, a pilot borehole heat exchanger was built in the main campus of the Universitat Politècnica de València. The present work shows the results of the experiments performed at the site, analysing the improvements that have been introduced both in the control of the heat injected during TRTs and in the methods to infer the ground thermal parameter. Three models are compared: two based on the infinite-line source theory and one based on the finite-line source scheme. The models were tested under two possible configurations of the equipment, i.e., with and without strict control of injected heat. Our results show the importance of heat injection control for a robust parameter assessment and the existence of additional heat transfer processes that the used models cannot completely characterize and that are related to the presence of significant groundwater flow at the site. In addition, our experience with the current installation and the knowledge about its strengths and weaknesses have allowed us to design a new and more complete test-site to help in the analysis and validation of new ground heat exchanger geometries.

A New Method Based on Thermal Response Tests for Determining Effective Thermal Conductivity and Borehole Resistivity for Borehole Heat Exchangers

“…According to the well-established Kelvin infinite line source theory thermal response test was developed [46]. The TRT methodology is based on the partial differential equation form of the Fourier thermal conductivity equation, which describes the dynamic dependence of temperature T on the distance from heat exchanger r and duration of the test t, i.e., determines T = T(r, t).…”

How Reliable Are Standard Thermal Response Tests? An Assessment Based on Long-Term Thermal Response Tests Under Different Operational Conditions

“…The test facility-completed in May 2010-is located inside the city of Valencia's UPV campus. All the information on the description of the installation is provided in Section 2.1 of Reference [45].…”

“…In 2010, our group started the construction of a research BHE to study soil thermal properties based on a fixed location at the university campus that serves as a reference site to compare acquisition and control methods, to proof models, or to evaluate the performance of different devices in exactly the same conditions (the description of the construction, its main characteristics, and initial tests at the site can be found in Reference [43]). During different runs of experiments, our site was used to understand and monitor the thermal stabilization time after grouting [43], to check the consistency between the soil parameters extracted from TRT test at different heat injection rates [44], and to analyze control strategies to minimize thermal influences from the environment [45]. This contribution focuses specifically on the study of groundwater flow effects among other factors affecting long-term TRT behavior and its quantification.…”

Investigation of Steady-State Heat Extraction Rates for Different Borehole Heat Exchanger Configuration from the Aspect of Implementation of New TurboCollectorTM Pipe System Design

“…The investigation and development of more precise methods for TRT control, modelling and evaluation in semi-permeable soils, with large water content, was examined by testing a pilot borehole heat exchanger [2]. The comparison of three models was preformed: two based on the ILS theory and one based on the finite-line source (FLS) scheme with the two equipment configurations; with and without strict control of rejected heat.…”