Full-process modelling and 4E analysis of an off-grid offshore green methanol production system for an energy island

Power-to-methanol systems offer a promising pathway for large-scale offshore renewable energy utilization, due to methanol's high volumetric energy density and safe liquid-phase storage and transport. However, studies on offshore green methanol synthesis systems remains limited, particularly comprehensive performance evaluations based on full-process models that account for offshore operating conditions. This study proposes an offshore wind-solar-wave hybrid green methanol production system for an energy island, integrating renewable power generation, hydrogen production, direct air capture of CO2, and green methanol synthesis. A rigorous full-process thermodynamic model and an energy management strategy were developed via cross-platform co-modelling framework. Energy, exergy, techno-economic, and environmental (4E) assessment was conducted using hourly full-process simulations over 8760 h to evaluate the system's comprehensive performance. To optimize the system's process design, inter-subsystem heat integration was implemented, and its quantitative impact on the system's 4E performance was assessed. At 500 MW of renewable capacity, the system delivers 7.51 × 104 tons/year of methanol with energy and exergy efficiencies of 9.02% and 13.81%, respectively; the levelized cost of methanol (LCOM) reaches 2708.4 $/ton, roughly an order of magnitude above the production cost of fossil-based methanol and 3–6 times the current market price. When idle offshore floating platforms are reutilized and carbon-pricing revenues from CO2 mitigation are considered, the LCOM decreases to 1921.3 $/ton, significantly improving economic competitiveness. Inter-subsystem heat integration enhances the energy and exergy efficiencies by 0.7% and 1.74%, respectively, and reduces the LCOM by 348.2 $/ton (11.39%). These results provide quantitative guidance for the large-scale deployment of offshore green methanol production systems on energy islands.