Renewable Energy–Hydrogen Integrated Energy Systems as Power-to-X Backbones: A Mini-Review of Topologies, Applications, Challenges, and Perspectives

This mini-review examines renewable energy–hydrogen integrated energy systems as power-to-hydrogen backbones for downstream power-to-X deployment. The review does not treat wind power, hydropower, microgrid operation, or power-grid dispatch as independent topics. Instead, renewable electricity sources and grid interfaces are considered only insofar as they affect hydrogen production, storage, reconversion, coproduct utilization, and downstream PtX compatibility. Representative hydrogen-centered topologies, including renewable energy–electrolyzer–market systems, renewable energy–electrolyzer–hydrogen-to-power systems, hybrid renewable energy systems, hydrogen-based hybrid energy storage systems, and combined heat and power coupled systems, are comparatively analyzed. Microgrids, offshore wind-to-hydrogen schemes, and regional deployment patterns are discussed as application boundaries for hydrogen systems rather than as separate power-system reviews. The analysis indicates that open-loop configurations are more suitable for supplying hydrogen to industrial, transport, and chemical end uses, whereas closed-loop hydrogen-to-power configurations provide long-duration flexibility and support renewable hydrogen integration where reconversion to electricity or heat is required. As electrolyzer technology, hydrogen storage, and downstream conversion routes mature, renewable energy–hydrogen systems are expected to evolve into scalable PtX-enabling infrastructure for low-carbon fuels, chemicals, and integrated energy networks.