A Comparison Between Traditional and Hybrid Optic Fibre Based Ground Thermal Response Tests

The importance of analysing and understanding the entity of the ground thermal conductivity aimed at evaluating the heat exchange capability in ground-source heat-pump (GSHP) applications is crucial. The subsoil represents the limiting factor in borehole heat exchanger (BHE) field design for building conditioning, due to its immutability and the cumbersome expenses associated with the installation drilling phase. Currently, there are several methods for assessing the thermal properties of a geological setting: laboratory analyses on sample corings, thermal response tests (TRTs) and distributed TRTs carried out with fibre optic sensors. In this regard, a GSHP system of more than 60 BHEs 120m deep has been realized at the new humanistic campus of the University of Padova (Italy). The coring provided a detailed stratigraphic sequence of unconsolidated alluvial deposits. Besides, the monitoring well has been equipped with a hybrid optical fibre cable integrating some electrical wires conductors and a bundle of fibre optics, sealed into the well. The fibre optic cable has been used here in an active mode to perform an Enhanced Thermal Response Test (ETRT) by injecting a constant heating power through the electrical wires contained within the cable structure and by measuring the transient thermal behaviour of the borehole. This kind of TRT has distributed features because exploits the optical fibre sensing technology to provide a spatial distributed representation of the behaviour of the subsoil along the stratigraphic succession. In the paper, the data acquired from the distributed ETRT have been analysed with two different method (analysis of the measured temperature by applying the first-order approximation of the infinite line-source model and the derivative analysis); the results are compared each other and to the global thermal conductivity provided by the traditional TRT in relation to the local stratigraphic succession.