Ground Heat Storage Systems: perspective in Mediterranean environments.

Geothermal energy is the more environmentally friendly energy source, continuously expanding between the renewable energies. Conditioning systems increased worldwide from about 4000 GW in 1990 to 11000 GW (and more) in 2016, and the energy consumption for space heating and cooling is expected to more than triple until 2050 (International Energy Agency, 2018). In particular, cooling down is catching on. As incomes rise and populations grow the use of air conditioners is becoming increasingly common, especially in commercial buildings and high-density residences of the hottest world regions, accounting for about a fifth of the total electricity in buildings around the world (International Energy Agency, 2019). This growing energy demand is in fact covered using refrigeration systems, which use electrically driven compressors to transfer energy. In this framework, low-carbon solutions can be a great possibility by ensuring clean energy without any form of pollution as well. A new frontier is covered by ground heat storage systems such as Underground Thermal Energy Storage (UTES) between the cheapest and efficient: cold energy can be captured and stored in the underground in the winter season (aquifers and/or soils and rocks) and can be provide cooling buildings in summer, and vice versa. By contributing to large-scale energy efficiency, long-term storage of thermal energy significantly reduces environmental impacts, increasing the potential uptake and recovering heat flows that are otherwise lost. Several examples in Canada and Northern Europe demonstrated the reliability and convenience of these systems in terms of both energy and economic savings, but more demonstration sites are however necessary. Usually numerical modelling is used to forecast the system behaviour but results of simulations can be strongly dependent from assumed material characteristics and should be strictly calibrated on real data. In order to better understand thermal processes in the ground related to thermal injection and thermal storage, a field scale BTES living lab is available as case study. It is located nearby Torino (NW Italy) within unsaturated alluvial deposits. It allows understanding possible differences at field scale, in order to evaluate and to monitor the ground behavior over time and the suitability of deeper resources. Results show that approximately 9.1 GJ were transferred to the ground during the first year, raising the undisturbed temperatures by 2°C, and that a correct comparison of monitoring data and numerical simulations can be obtained following a specific site characterization.