Dynamic Safety Boundary Modeling and Flexibility Assessment of Alkaline Electrolyzers Under Fluctuating Wind and Solar Conditions

Alkaline water electrolysis (ALK) is essential for renewable energy integration, yet traditional models using a fixed minimum operating power often overestimate low-load flexibility by neglecting state-dependent safety boundaries. This study develops an electro-thermal-mass multiphysics dynamic model that treats the transient hydrogen content in oxygen (H2-in-O2) concentration as a first-principles state variable. Based on a quasi-steady-state safety balance, a dynamic minimum operating power constraint is derived to replace empirical static limits. A key feature of this model is the explicit coupling of Arrhenius thermal diffusion and pressure-differential-driven permeation during load transients, allowing the safety threshold to respond to real-time system states. Year-round simulations of a 30 MW industrial system under a wind–solar time series reveal that thermal inertia, with a time constant of approximately 4.2 h, induces a sustained mismatch between low-power operation and high system temperature. Under high-temperature and rapid-ramp conditions, the dynamic safety lower bound reaches 28.2% of the rated capacity, exceeding the conventional 20% static threshold by 8.2 percentage points. This deviation results in 378.3 MWh of implicit curtailment and 60.5 h of additional downtime annually, leading to a green hydrogen production deficit of approximately 42.2 t/year. This research provides a theoretical foundation and technical reference for the optimal control and flexibility assessment of industrial-scale green hydrogen systems under fluctuating power supply conditions.