Hydrological constraints on pumped-storage hydropower shape solar expansion potential in Northern Italy

Italy hosts the largest pumped-storage hydropower (PSH) capacity in Europe, yet PSH utilization has declined sharply since Italian energy market liberalization occurred in 2006, despite no observed decrease in water availability. With solar photovoltaic (PV) capacity projected to triple by 2030 under Italy's decarbonization targets, indeed this underutilized infrastructure could provide critical flexibility for absorbing renewable surpluses. This raises a key question: What is the hydrological potential of existing PSH plants to absorb surplus generation from variable renewable energy sources regardless of market incentives? Using HYPERstreamHS, a large-scale hydrological model, we quantify this absorption potential at the plant level across the Italian Alpine Region. Our simulations indicate an average absorption capacity of 5.4 TWh yr -1, limited by water availability, of which only 12% was utilized in 2024. Under a sequential surplus-handling approach that considers the simulated PSH potential as an upper-bound threshold, Italy's Northern bidding zone can integrate solar PV generation up to 270% of 2024 levels before reaching system saturation. This corresponds to approximately 35.7 TWh yr -1, raising the renewable share of local electricity production from 46% to 62%. Beyond saturation, approximately 1 TWh per year of surplus energy at full projected deployment would require curtailment or alternative flexibility measures. These findings demonstrate that substantial solar expansion is achievable with the existing PSH infrastructures and that current PSH underutilization is primarily economically driven rather than physically constrained. The proposed framework provides a transferable approach for assessing storage potential limited by water availability in Alpine regions facing increasing renewable integration.