Review of sizing methodologies for hydropower-based hybrid energy systems: a techno-economic perspective

Hydropower-based hybrid energy systems (HBHES) serve as a cornerstone for integrating intermittent renewables and ensuring grid resilience, yet unlocking their full potential depends on optimal capacity sizing. Unlike standalone wind or solar plants, HBHES sizing couples long-term capacity planning with short-term operational and hydraulic constraints. This paper presents a comprehensive techno-economic review of HBHES sizing, synthesizing recent studies within a unified framework that covers system configurations, treatment of uncertainties, techno-economic indicators, and optimization methods. Four main findings emerge: first, a clear hierarchy of uncertainties exists: hydrological variability (inflow) and electricity prices are the dominant drivers of optimal capacity. Second, sizing decisions are governed by nonlinear mechanisms (marginal effects and techno-economic trade-offs), so pushing reliability metrics, such as the power curtailment rate, beyond a critical threshold yields diminishing marginal effects and disproportionate increases in cost. Third, current evaluation practice relies heavily on plant-level levelized cost of electricity and aggregate loss of power supply probability, while rarely accounting for system-level integration costs, negative prices, extreme “energy droughts” or differentiated reliability needs for critical loads. Fourth, metaheuristic, mixed-integer linear programming-based, and artificial intelligence methods occupy complementary positions in an optimization workflow, although these model and algorithmic errors, as well as computational efficiency trade-offs, are seldom systematically evaluated. Based on these insights, the review proposes a research roadmap emphasizing integrated uncertainty modeling, system- and resilience-oriented metrics, hybrid optimization with explicit error analysis, and the integration of emerging socio-environmental considerations such as brownfield pumped hydro storage development and off-grid rural electrification.