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