Hydrogen valleys: Advancing the hydrogen economy through decarbonization, energy flexibility, and sector coupling

To support the energy-system transition toward climate neutrality, scalable solutions that enable renewable electricity integration, multi-sector coupling, flexibility, and carbon-emission reduction are increasingly required. However, no existing review provides a cross-project assessment of hydrogen valleys as an integrated deployment model. This review addresses this gap by developing an Integrated Hydrogen Valley Systems Framework grounded in Industrial Ecology, Energy Hub theory, and Technological Innovation Systems theory. Using an evidence-based approach, we systematically analyse all 90 hydrogen valley projects listed in the Clean Hydrogen Partnership's H2V database. Peer-reviewed studies, official project databases, technical reports, and policy documents are synthesised to compare renewable-energy integration pathways, electrolyser technologies, hydrogen storage and transport options, sector-coupling strategies, and supporting policy frameworks. The findings indicate that hydrogen valleys can facilitate the integration of wind and solar power, enhance energy-system flexibility, enable long-duration energy storage, and contribute to carbon-emission reductions of up to 30%, with a levelised cost of hydrogen of approximately €5.6/kg under an optimal sector-coupling scenario. Proton exchange membrane and alkaline electrolysers account for most deployments, particularly in Europe. Nevertheless, project development remains constrained by renewable-resource variability, high capital costs, limited infrastructure, regulatory gaps, and supply-chain constraints. The review identifies key policy priorities, including technology-neutral investment support, harmonised cross-border hydrogen-trade frameworks, accelerated permitting, and expanded electrolyser manufacturing capacity, to strengthen the scalability and global development of hydrogen valleys.