Comparison between DTRT and GEOsniff TRT methods for thermophysical characterization of the ground

The research is oriented to provide elements of knowledge, useful for the choice of new generation methods of in situ thermal characterization of the subsoil, which may be more suitable to provide quantitative information on the design and sizing parameters for closed-circuit heat exchange systems. In particular, it is essential to determine the thermal resistance of the ground heat exchanger as well as the ground undisturbed temperature and the thermal conductivity of the ground affected by the heat exchange processes, being able to differentiate the contributions of the different geological levels crossed by the ground heat exchanger. These on-site tests are necessary to design the system and determine the thermal characteristics of cement mixtures to be apply to finally sealing the wells in which the ground heat exchangers are installed.

This paper analyzes and compares the results obtained from two different on-site thermal response test methods, carried out to design a borefield of a GSHP plant, foreseen in the renovation project of the Swiss Embassy in Italy in Rome (RM). The boreholes reach a depth of 200m and are installed in a geological context characterized by volcano-clastic deposits. In particular, two methods of distributed investigation of subsoil temperature and thermal properties are compared. The two methods are based on different technologies: optical fiber heating cable (DTS) and an innovative method that uses smart spheres ("GEOsniff"- enOware) floating inside the boreholes, integrated with miniaturized electronics, able to measure the temperature of the water inside the tube all along the whole vertical.

 

Both tests were carried out closed in time and in the same borehole heat exchanger. In particular, the fiber optic DTS (Distributed Temperature Sensing) technology, based on hybrid fiber optic cable and heating wire, performs a distributed thermal response test (DTRT) in one of the vertical geothermal exchangers. The "enOware GEOsniff" technology, based on special sensors and heating cable, was realized 2 months after, to perform a second advanced distributed thermal response (E-TRT) test in the more than one vertical geothermal exchanger.

In addition, thermal conductivity measurements and laboratory characterizations were carried out on 4 samples acquired from the corings derived during the drilling operations, thus providing specific values of thermal conductivity of the different levels.

The thermal conductivity values obtained in different ways are directly compared and discussed.