Potential and limits of the Po Plain for the development of open-loop Groundwater Heat Pump Systems (GWHPs) in Italy, the case study of Milan and Turin

Italy plans to pursue the target of obtaining 30% of final consumption of energy from renewable energy sources (RES) in 2030, by defining a pathway of sustainable growth for RES and the full integration thereof into the system. In this context, Groundwater Heat Pump systems (GWHPs) represent one of the most suitable technologies to be applied in the heating and cooling of buildings in densely urbanised areas, reducing CO2 emissions and environmental pollution. Currently, the share of geothermal heat production, out of the total thermal production from Renewable Energy Sources (RES) in Italy, is limited to 2.1%. It is, therefore, necessary to incentivise and encourage the deployment of geothermal solutions in order to increase the RES percentage and reach the European targets. However, different environmental aspects must be considered to minimize the impact of GWHP systems on the subsurface and shallow aquifers. Therefore, urban planning tools must pursue a rapid deployment of GWHP and ensure adequate long-term protection of the groundwater bodies, through an understanding of the subsoil in the decision-making process. A proper geological and hydrogeological characterization is fundamental and required by regulatory authorities for allowing the correct development of GWHPs. To date, the numerical model is the most powerful predictive tool, and a model calibration is required to perform uncertainty analyses, by connecting the performed model to the real world. For developing realistic numerical models, a set of data is necessary to properly define the fundamental hydrogeological parameters, such as horizontal hydraulic conductivity (Kx, Ky), porosity (η), longitudinal dispersivity (α), and storativity (S). Two case studies were considered: Turin City (Piedmont Region), where more than 50 authorisations for geothermal discharges have been submitted, and Milan City (Lombardy Region), where there are more than 200 geothermal plants. Despite the similar hydrogeological contexts (presence of unconfined, highly productive aquifer), the analysed cities are characterized by different open-loop diffusion rates. The in-force regional and municipal regulatory references to which a new geothermal project must comply (i.e., authorization requests and plant final testing operations) were taken into consideration, highlighting the potential connected to the diffusion of GWHPs and understanding the causes of the different diffusion rate in the mentioned contexts.