GNGTS 2013 - Atti del 32° Convegno Nazionale

correct monitoring of these systems besides local temperature measurements carried out in the thermal exchangers themselves. Basically, two types of geothermal systems use the ground as a heat source - open and closed loop systems. Both open and closed loop systems are generally coupled with electric heat pumps which arise the achieved temperature with a thermodynamic cycle. On the one hand, the open loop systems (aquifer thermal systems) use the shallow groundwater as warm or cold source directly. The groundwater circulates within one or more extraction and injection wells. In summer periods, the groundwater is used to cool down the building and the heat is transferred into the ground, with an increasing groundwater temperature approximately from 6 to 8 °C (Bonte et al. , 2011). In winter periods, the groundwater gives the heat necessary to warm up the building, with a consequent cooling effect on the groundwater temperature of approximately 5 to 10 °C being observed (Bonte et al. , 2011). On the other hand, the closed loop systems exploit the ground as a heat source. The borehole heat exchangers (BHEs) used by these applications are conductor pipes filled with different heat transfer fluid. The circulating fluid within these pipes extracts the heat from the ground and thereby it influences the ground temperature indirectly. Similarly, other shallow geothermal applications exploit the ground as a storage volume for the thermal energy. These systems are generally called Seasonal Thermal Energy Storage (STES), because they transfer the heat in the ground in the summer periods and they extract it when the heating demand arises, in the cold periods (Xu et al. , 2013). The STES systems include several methodologies for storing the heat: for instance the ground is used as storing medium and the connection with the ground is provided by a series of BHEs (Reuss et al. , 2006). In all of the mentioned applications a wide knowledge of the thermal properties of the ground and their influencing factors is needed towards a reliable design, which should be efficient, cost-effective and environmentally sustainable. Moreover, some correct monitoring protocols are required to evaluate the efficiency of the system and prevent environmental problems. This study aimed at evaluate at laboratory scale the use of non-invasive electrical measurements for the characterization and monitoring of shallow geothermal applications. Electrical measurements could be indeed considered as a time and cost efficient method for the characterization and long-term monitoring of shallow geothermal systems. We have evaluated their potentiality in monitoring groundwater and soil changes under temperature variations by performing laboratory measurements under known boundary conditions in an ad hoc designed thermal box and to analyze the correlation between electrical and thermal properties for different media, taking their influencing parameters into account. This can be viewed as a fundamental calibration step for a further application of the methodology to a real case study. Theoretical background. State of the art . So far, there is limited specific knowledge about the long-term effects of unsuitable system design or the effects of groundwater temperature changes and chemical changes within the subsurface and the resulting consequences. A few studies have already measured the thermal effects of geothermal plants within field sites, for instance Huber and Arslan (2012). They also compared their field results with numerical simulations and laboratory measurements with a forced groundwater flow (Arslan and Huber 2013). Nevertheless, these studies do not fill the criteria of being rapid, non-invasive and in-expansive monitoring technologies for efficient applications and environmental protection. Geophysical methods can conversely provide information over large areas at relatively inexpensive costs compared to other general methods or applications. For example the surface electrical resistivity tomography (ERT) has many practical applications for studying soil properties and processes in the subsurface (Ramirez et al. , 1993). In the context of geothermal reservoirs, ERT has already been extensively applied for hydrothermal fluids. For temperatures up to 150 °C, it reveals characteristic specific resistivity values in contrast to their corresponding soil resistivity values. Therefore, with the ERT method, the geothermal 123 GNGTS 2013 S essione 3.2