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.
Highlights
The characterization of ground thermal properties using in situ thermal response tests (TRTs)[1] is a common procedure for designing HVAC systems based on geothermal heat pumps
TRT is based on the thermal response of a BHE to a steady, several days long, heat injection, or extraction pulse [2]
The most relevant variables measured in a TRT are the temperature of the heat carrier fluid at the input and output of the heat exchanger, taken throughout the execution of the test
Summary
The characterization of ground thermal properties using in situ thermal response tests (TRTs). The most relevant variables measured in a TRT are the temperature of the heat carrier fluid at the input and output of the heat exchanger, taken throughout the execution of the test Comparing these experimental data against a model that describes heat transfer between the fluid and the ground, thermal properties of the ground can be estimated. The system was improved by introducing a Proportional–Integral–Derivative (PID) controller to keep the injected heat rate constant, independent of the influence of ambient temperature This has enabled validation of new or modified BHE models [5] and to verify the performance improvement of other models with minimum modification of the installation. This paper presents the comparison of the data obtained in the first TRTs performed without controlling the injected power with those recorded in the tests carried out with a PID control that regulated the power injected in the borehole
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