Off-grid green hydrogen production and liquefaction system driven by renewable energy and LNG cold energy: A comprehensive 4E analysis and optimization

To address the growing demand for sustainable hydrogen production and reduce the carbon footprint of hydrogen liquefaction, an off-grid system integrating renewable energy, liquefied natural gas cold energy and organic Rankine cycle is proposed. The renewable energy generation and proton exchange membrane electrolyzer hydrogen production processes are modeled in TRNSYS, while the hydrogen liquefaction and the organic Rankine cycle are simulated using ASPEN HYSYS. The Particle Swarm Optimization algorithm is used to optimize the hydrogen liquefaction process by evaluating various configurations based on energy efficiency, environmental impact, exergy efficiency, and economic feasibility. The optimization results show that the system achieves a reduction in specific energy consumption from 7.948 $\mathrm{kWh}/{\mathrm{kg}}_{{\mathrm{LH}}_2}$ to 6.937 $\mathrm{kWh}/{\mathrm{kg}}_{{\mathrm{LH}}_2}$. Within the 4E analytical framework, Case 10 achieves the highest energy efficiency at 23.55 %, whereas Case 1 demonstrates the most significant pollutant reduction, decreasing emissions by 2.901 % relative to the reference system. Case 11 exhibits the best exergy efficiency at 26.61 %, while Case 8 optimizes economic viability with the lowest initial investment, featuring a dynamic payback period of 3.56 years and a levelized hydrogen production cost of 1.1 $/${\mathrm{kg}}_{{\mathrm{LH}}_2}$. From a dual-criterion perspective, Case 8 outperforms others in life cycle cost and emission reduction performance, while Case 10 maintains superior energy and exergy efficiency. Significantly, Case 5 emerges as the Pareto-optimal solution under equally weighted multi-criteria evaluation, balancing all performance indices with minimal trade-off compromise. This integrated system provides a promising solution for utilizing offshore renewable energy in hydrogen production, offering a low-emission and sustainable fuel pathway.