Low enthalpy geothermal systems: coupled flow and heat transport modelling of the long-term performances of Borehole Heat Exchangers

The Ground Source Heat Pump (GSHP) is a promising technology for the heating and cooling of buildings with renewable energy sources. Borehole Heat Exchangers, in which heat is exchanged by circulating a heat carrier fluid into a pipe closed loop, are the most used typology. The energy performances of these plants depend from the properties of the BHE and of the soil. In this work, the operation of BHE for a period of 30 years has been simulated, using a finite-element subsurface flow and heat transport modelling code (FEFLOW). The relative influence of each BHE, hydrogeological and thermal soil property has been investigated, running a set of simulations and analyzing the resulting fluid temperatures in the closed loop to estimate the heat pump energy consumption. Comparing the results, we observe that the length is the most important BHE property, and it should be optimized in order to minimize the overall expense (installation and maintenance); also the pipe distance, the grout heat conductivity and the properties of the heat carrier fluid play an important role. The soil heat conductivity heavily influences the resulting fluid temperatures, and in-situ tests should be carried to predict the plant operation accurately. The presence of subsurface flow enhances the heat transfer in the subsoil, improving the heat pump performances. The heat dispersivity spatial dependence is still unknown, and this causes a strong uncertainty in the prediction of BHE operation in presence of groundwater flow.