NUMERICAL AND EXPERIMENTAL ANALYSIS OF SOLAR ASSISTED BOREHOLE THERMAL ENERGY STORAGE SYSTEMS

An analysis of 2022 data reveals that over 50% of the energy consumed in buildings is used for heating and hot water production. Furthermore, two-thirds of this energy is still generated using fossil fuels. In this context, the use of heat pumps for heating buildings, if powered by renewable electricity, could enable greater sustainability in this sector All these systems can be also integrated with solar thermal systems, in particular, ground-source heat pumps can extract heat in winter from borehole thermal energy storage produced by solar thermal collectors in the previous summer. In some cases, for example when there is little space available, these collectors can be made using road thermal collectors. In this PhD thesis, new experimental and numerical methods were developed to analyze this type of plant from an energy point of view. These methods were validated using measurements recorded in monitoring campaigns performed of a new pilot plant (SMARTEP) built at a car park on campus of University of Palermo. This plant integrates a road thermal collector and a borehole thermal energy storage and aims to demonstrate the feasibility of this type of system for heating non-residential buildings in the Mediterranean. The new method involved the construction of a pilot borehole heat exchanger and a pilot road thermal collector, both instrumented with temperature sensors, and the execution of non-conventional thermal response tests using heating cables. The interpretation of the experimental data was performed using back analysis techniques based on analytical and numerical finite element models, allowing the characterization of the thermal conductivity and diffusivity of the different soil layers and materials constituting the solar collector. In the process of this work, a new numerical model was also developed to simulate the short- and long-term response of the borehole heat exchanger. In addition, three-dimensional models of the storage and two-dimensional models of the solar collector were developed. The former were used to define g-functions of the storage and the latter were validated with a series of thermal response tests performed on the pilot plant. Finally, a numerical model of the entire SMARTEP plant was developed for the aim of simulating dynamic operation during the first 5 years of the plant's life. From results of these simulations, assumed values of storage efficiency of the new system were assessed.