Scaling net-zero carbon and firm power via co-optimizing power-to-gas and gas-to-power for seasonal storage and curtailment elimination in isolated microgrids

• Clarifying green hydrogen seasonal storage role in emission-constrained hybrid energy systems. • Developing an 8,760-hour MILP framework co-optimizing PtG, GtP, and system sizing and operation. • Identifying economic critical points where hydrogen becomes essential under near- and net-zero CO 2 limits. • Hydrogen cuts near-zero system costs by 5–50%, while net-zero without it raises costs above 110%. This paper presents a fully linear, year-long optimization framework for the joint sizing and hourly operation of a hybrid renewable microgrid comprising solar PV, wind power, battery storage, and a complete Power-to-Gas/Gas-to-Power hydrogen chain. The formulation preserves device-level physics through linearized representations and enables the simultaneous assessment of short-duration (battery) and seasonal storage (hydrogen) dynamics. When applied to a representative isolated microgrid, the inclusion of the hydrogen chain reduces curtailed renewable energy considerably, thereby increasing the capacity factors of PV and wind resources. Seasonal hydrogen storage exhibits two extended charge/discharge cycles, providing a firm supply during prolonged renewable deficits. Compared with configurations without hydrogen, near-zero-emission scenarios show 5–50% lower system cost, while achieving net-zero emissions without hydrogen requires an over 110% cost increase, even when limited load shedding is permitted. These results indicate that hydrogen-enabled seasonal storage substantially enhances renewable energy deployment and system feasibility under stringent emission constraints.