Borehole Heat Exchanger simulations in aquifer: the borehole grout influence in thermal response test modeling

Geothermal energy, and especially the use of low enthalpy resources, has a rising importance; ground-coupled heat pump (GCHP) systems have been used increasingly because they are among the cleanest and most energy efficient heating and cooling systems for buildings. Simulation models can be applied for a more effective use of the subsoil for geothermal purposes. In fact they are useful tools for the design of efficient systems which consider also the need to avoid abnormal temperature distributions in soil and aquifers. In the hydrogeology field the Modflow/MT3DMS codes are the most widely used programs to face environmental problems and forecast quantity and quality impacts on groundwater resources. Although Modflow/MT3DMS are used to represent open circuit heat pumps, they had never been used to represent borehole heat exchangers (BHE). Aim of this study is to simulate a Thermal Response Test (TRT) through Modflow/MT3MS codes implementing into the model all the components of a Ground Heat Exchanger system: from the U-shaped BHE to the grout material surrounding it. For GCHP systems, the TRT is commonly used to determine the heat transport parameters of the subsurface as thermal conductivity and thermal diffusivity. It also allows to determine the BHE thermal resistance which mainly depends on the geometry of the dug area and on the thermal properties of the surrounding grout material. Starting from a model implemented in a previous work (Angelotti et. al., 2014) Modflow has been used for the simulation of a TRT. Two cases have been analysed and compared: in the first the cells around the BHE are assigned the aquifer parameters while in the second they have been assigned the characteristics of the grout material (hydraulically impermeable but highly conductive from the thermal point of view). A 10-6 m/s groundwater Darcy velocity case is analysed applying a 40 W/m specific heat rate to the fluid circulating into the BHE. Considering the temperature distribution in the ground, the differences between the two simulated cases (with/without-grout) result essentially negligible presenting a maximum value equal to 0.07 °C in the proximity of the BHE (around 20 cm from the centre of the U-pipes). A slightly more relevant difference (0.55°C) in simulated temperature is registered very close to the BHE, into the area occupied by the grout (only 6 cm from the centre of the U-pipes). This is due to the absence of the advective term in the zone where the grout is present and the heat transfer is linked just to conduction term. By the way there are almost no differences in the temperature of the heat-carrier fluid in the two cases (0.02 °C). These results lead to conclude that, simulating a TRT, the effect of the grout is negligible, therefore is advantageous to avoid modeling its geometry and thermal/physical parameters so as to save time and energies in the model implementation phase