High photovoltaic (PV) penetration creates midday surplus generation, steep evening ramps, and persistent renewable curtailment, increasing the need for coordinated short- and long-duration flexibility resources. This paper proposes a two-stage mixed-integer planning framework for battery energy storage systems (BESS) and hydrogen energy systems (HES) in a PV-dominated transmission network. In the first stage, the hydrogen subsystem is optimized independently to determine the baseline electrolyzer schedule and the associated hydrogen infrastructure required to satisfy hydrogen demand. In the second stage, the power system and hydrogen system are co-optimized. Demand-response compensation is applied only to downward deviations from the baseline electrolyzer schedule, while shifted load, newly installed electrolyzer capacity, newly added hydrogen storage, and fuel-cell operation are not compensated as demand-response resources. The model includes DC network constraints, PV generation, battery storage, hydrogen transportation, and degradation of batteries, electrolyzers, and fuel cells over a 25-year planning horizon represented by four seasonal days. The framework is applied to the IEEE 24-bus system with six 400 MW PV plants. In the degradation-aware setting, the HES-only flexibility case reduces total cost by 10.4% and PV curtailment by 65.0% relative to the base case but increases emissions by 27.0%. The coordinated hybrid BESS-HES case provides the best overall performance, reducing total cost by 15.7% and PV curtailment by 90.5%, while limiting the emissions increase to 6.3% relative to the base case and lowering emissions by 16.3% relative to the HES-only case. The results also show that degradation shifts the optimal battery design toward lower power and higher energy capacity and reduces electrolyzer degradation cost by 47.3% in the hybrid case compared with the HES-only case. These findings demonstrate that baseline-constrained hydrogen demand response provides a more realistic basis for long-term PV-dominated grid planning than treating all hydrogen flexibility as uniformly compensable.

Keywords

Hydrogen energy systems, demand response, battery energy storage, power system planning, process optimization.