Comparison between traditional and enhanced Thermal Response Test for ground thermal properties estimation
Authors: Galgaro Antonio , Carrera Alberto , Di Sipio Eloisa 
Published: March 2021

&lt;p&gt;For the design and implementation of an efficient Ground Source Heat Pump (GSHP) system, the local&lt;br&gt;subsoil represents the core element. Since the thermal performance of Borehole Heat Exchangers (BHEs) is&lt;br&gt;site-specific, its planning typically requires the knowledge of the thermal proprieties of the ground, which&lt;br&gt;are influenced by the local stratigraphic sequence and the hydrogeological conditions. The evaluation of&lt;br&gt;the variations of the ground thermal conductivity (TC) along the depth, as well as its undisturbed&lt;br&gt;temperature, are essential to correctly plan the BHEs field and improve the performance of the ground&lt;br&gt;heat exchangers themselves.&lt;br&gt;Thermal Response Test (TRT) is a well-known experimental procedure that allows to obtain the thermal&lt;br&gt;properties of the ground. However, the traditional method provides a single value of the equivalent TC and&lt;br&gt;the undisturbed temperature, which can be associated with the average value over the entire BHE length,&lt;br&gt;with no chance to detect the thermo-physical parameters variations with depth and to discriminate the&lt;br&gt;contributions of the different geological levels crossed by the geothermal exchange probe. Indeed,&lt;br&gt;different layers within a stratigraphic sequence, may have different thermal properties, according to the&lt;br&gt;presence and to the flow rate of groundwater, as well as to granulometry and mineralogical composition,&lt;br&gt;density, and porosity of the lithologies. The identification of the different contributions to the thermal&lt;br&gt;exchange provided by each geological unit, in practice, can further support BHE design, helping to&lt;br&gt;determine the most suitable borehole length and number, achieving the highest heat exchange capability&lt;br&gt;at the lower initial cost of implementing of the entire geothermal plant.&lt;br&gt;In the last years, new improved approaches to execute an enhanced thermal response test have been&lt;br&gt;developed, as the pioneer wireless data transmission GEOsniff technology (enOware GmbH) tested in this&lt;br&gt;study. This measurement method is characterized by its sensors, 20mm-diameter marbles equipped by&lt;br&gt;pressure and temperature transducers combined with a system of data storing and wireless data&lt;br&gt;transmission. Released at regular intervals down the testing BHE, infilled with water, each marble freely&lt;br&gt;floats allowing the measurement of the water temperature variations over time at different depths, in&lt;br&gt;order to identify areas with particular values of thermal conductivity related to distinctive hydrogeological&lt;br&gt;conditions or lithological assessment. This way, the GEOsniff technology allows a high-resolution spatially-&lt;br&gt;distributed representation of the subsoil thermal properties along the BHE.&lt;br&gt;In this work, we present the test outputs acquired at the new humanistic campus of the University of&lt;br&gt;Padova, located in the Eastern Po river plain (Northern Italy). The thermal conductivity data obtained by&lt;br&gt;the GEOsniff method have been compared and discussed, by considering the standard TRT outputs. This&lt;br&gt;innovative technique looks promising to support the optimization of the borehole length in the design&lt;br&gt;phase, even more where the complexity of the treated geological setting increases.&lt;/p&gt;