The role of acquifer characteristics in the thermal performance of different borehole heat exchanger configurations: a case study in NW Italy

The use of renewable energies as an integration in multivalent district heating and cooling networks

(DHC) has been growing in recent years and a few systems are already operative across Europe.

A proper design is of paramount importance to guarantee the energy performance of the system. This work deals with the optimization of the technical and geometrical characteristics of borehole heat exchangers (BHE) in a well-defined hydrogeological context, aimed at the integrating the space heating and cooling of buildings. The test site is NW Italy where a gas-fired DH grid is currently operating. Three different configurations were analysed by investigating their thermal performances according to available geological information that revealed an aquifer in the first 36 m, overlying a poorly permeable marly succession. Numerical simulations were used to validate the geological, hydrogeological, and thermo-physical models by back-analysing the experimental results of a Thermal Response Test (TRT) on a pilot 150 m deep BHE. Five-years simulations were then performed to compare 150-m and 36-m polyethylene 2U, and 36-m steel coaxial BHEs. Results show higher thermal power extractions in the shallower 2U BHE (56.03 W/m) compared to the deeper one (42.47 W/m), probably due to presence of the aquifer which surely plays an important role in increasing the thermal power. The coaxial configuration shows the best performance both in terms of specific power (74.51 W/m) and borehole thermal resistance (0.02 mK/W). Outcomes of the study confirm that finding the best coupling between the geological framework and technical requirements, ensure the best energy performance and economic sustainability.