LNG Cold Energy for Hydrogen Liquefaction—A Critical Engineering Review

Hydrogen liquefaction is essential for large-scale hydrogen storage and long-distance transport, but it remains one of the most energy-intensive processes in the hydrogen value chain. Liquefied natural gas (LNG) regasification provides a valuable source of cryogenic exergy that can reduce the refrigeration demand of hydrogen liquefaction through process integration. This review critically examines the development of LNG-assisted hydrogen liquefaction from early conceptual studies to recent optimized, infrastructure-integrated, and multi-energy configurations. Published architectures are organized through a functional taxonomy comprising seven integration functions, which are treated as architectural building blocks rather than mutually exclusive process categories. Reported energy, exergy, and economic performance is critically assessed, showing that numerical indicators are strongly dependent on system boundaries, LNG operating conditions, plant scales, heat-exchanger assumptions, ortho-to-para conversion modeling, equipment performance, and economic assumptions. A three-level comparability framework is proposed to distinguish directly comparable, conditionally comparable, and contextual cross-study assessments. The review further identifies dynamic operations, safety, equipment scale-up, experimental validation, and site-specific economics as key barriers to industrial deployment. Future progress should therefore shift from increasingly optimized steady-state flowsheets to engineering-constrained design, dynamic validation, pilot demonstration, and integrated cryogenic-terminal deployment.