Hydropower as a platform for multi-energy hybrid systems: a review of solar, battery, and hydrogen integration

The properties of hydropower such as flexibility and controllability position it as a central platform for integrating variable renewable resources. This review synthesises research on hydropower–centric hybrid systems combining Floating Photovoltaics (FPV), power–type energy storage represented primarily by Battery Systems (BESS), and green hydrogen (H2). Pumped hydro energy storage (PHS) is included as a closely related extension of hydropower infrastructure to clarify the range of storage functions across the system. The technologies perform distinct and complementary roles, though some functional overlap exists in medium–duration energy shifting and diurnal balancing: FPV increases generation capacity while limiting reservoir evaporation; BESS delivers fast–response services; hydrogen supports long–duration and seasonal balancing; and PHS provides short– to medium–term bulk storage. A comparative analysis of techno–economic models, operational strategies, and case studies reveals that existing literature remains focused on pairwise or tri–hybrid systems, with a notable absence of comprehensive evaluations of fully co–optimised quad–hybrid (hydro–FPV–BESS–H2) architectures. Beyond this gap, the synthesis yields two original cross–study findings: modelling tool choice systematically biases conclusions on battery integration, and a scale tension in the hydrogen evidence leads to opposite investment recommendations at plant and grid levels. A diagnostic of five structural barriers explains why co–optimised quad–hybrid models have not materialised. Conditional design principles and a configuration selection framework show that the quad–hybrid is a boundary–condition–driven outcome rather than a universal solution. The review identifies priority research needs, including multi–timescale optimisation frameworks that capture hydraulic constraints, standardised techno–economic reporting, and empirical validation from operational installations. Overall, hydropower–anchored hybrid architectures show strong potential as a pathway toward resilient, high–renewable energy systems.