Thermal impact in aquifers and energy performance evaluation of borehole heat exchanger by the use of numerical modelling

Ground-Source Heat Pump (GSHP) are among the cleanest and most energy efficient systems for heating or cooling buildings as alternative to traditional methods. The study verified the possibility both to use GSHP systems in Zootechnical area and to numerically model these systems for thermal impact evaluations in aquifer and energy performances. A numerical model of a U-shape pipe of BHE in a saturated sandy porous medium was implemented in MODFLOW/MT3DMS codes, although this implied strong grid refinement and extensive computational resources. The first goal of the modelling phase was to simulate a BHE and compare its results with an analytic solution (generally used for thermal response test interpretation), known as linear source, for a case where groundwater flow was neglected (Gehlin et al., 2002). In most cases the groundwater flow influence on the energy performance of BHEs needs to be assessed and a Moving Line Source (MLS) solution was proposed by Molina Giraldo et al. (2011) in order to examine the interaction between BHE and groundwater flow. The temperature distributions in aquifer, outputs of the numerical model run in a constant heat rate mode, was then fitted with respect to MLS solution and the results proved that the numerical model was sufficiently accurate (Angelotti et al., 2014). After the validation, the numerical model was adapted to a constant inlet fluid temperature case, in order to reproduce a typical operation of a GSHP. In particular, the thermal-carrier fluid was circulated into U-pipes of BHE, extracting/injecting heat from/into the ground. The results achieved in these simulations concerned predicted exchanged energy and temperature distribution: groundwater flow velocity, varied to represent a wide ensemble of hydrogeological systems, and thermal dispersivity coefficient (advection and dispersion terms usually neglected by most of energy codes) played an important role in the heat transfer (Angelotti et al., 2014). At last, the numerical model was adapted to a real case, EcoZoo project, funded by Lombardy Region and Italian Ministry of Research and Education and supervised by Tethys Srl. The system was composed of five BHEs 60 meters deep, a heat pump, an air handling unit with heat recovery system; it provided heating, cooling and air exchange to a piglet room in Experimental Didactic Zootechnical Centre of the University of Milan. The monitoring system, set up in LabVIEW (an object base programming code), comprised the measurements of a set of different parameters, concerning energy and temperature in subsoil, to enable the numerical modelling of the environmental impact on groundwater system. The acquired parameters allowed the validation of numerical model through the comparison with the monitored data. Through the calibration of hydraulic and thermal parameters, a good agreement between numerical and real data was found. Therefore, the numerical model correctly reproduced the heat transfer in aquifer. It was also possible to thermally typify the aquifer and to reproduce the real length of thermal plume, after a heating operation. The presence of experimental data allowed to evaluate the capability of MODFLOW/MT3DMS, providing for the lack of experimental data in literature.