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| liquid hydrogen | |
|---|---|
| Name | Liquid hydrogen |
| Formula | H₂ |
| Molar mass | 2.016 g·mol⁻¹ |
| Appearance | colorless liquid |
| Density | 70.85 g·L⁻¹ (at 20 K, 1 atm) |
| Melting point | 14.01 K |
| Boiling point | 20.271 K |
| Critical temp | 33.145 K |
| Critical pressure | 12.8 atm |
| Heat of vaporization | 0.904 kJ·g⁻¹ |
| CAS number | 12385-13-6 |
liquid hydrogen Liquid hydrogen is the cryogenic liquid form of the diatomic molecule H₂. It is produced and handled at temperatures near 20 K and is notable for its extremely low density, high specific energy, and central role as a propellant and industrial feedstock. Because of these characteristics, it features prominently in aerospace, research, and emerging clean-energy systems.
Liquid hydrogen exists in two nuclear-spin isomers: ortho-hydrogen and para-hydrogen, with equilibrium conversion influencing thermophysical behavior relevant to National Aeronautics and Space Administration and European Space Agency cryogenic systems. Its boiling point (20.271 K) and melting point (14.01 K) place it among the lightest cryogens alongside liquid helium used at CERN and in Large Hadron Collider subsystems. Low density (≈70.85 g·L⁻¹) and high gravimetric specific energy make it attractive to organizations such as SpaceX and historical programs like Saturn V. Thermal conductivity and viscosity at cryogenic temperatures affect design in facilities operated by Air Liquide and Linde plc; impurities such as gaseous helium or neon change vapor pressure behavior relevant to Jet Propulsion Laboratory testbeds. Spectroscopic signatures in the infrared and Raman regions are exploited by teams at National Institute of Standards and Technology and MIT for diagnostics, while phase-change heat transfer influences tank insulation designs used by United Launch Alliance and research at Lawrence Livermore National Laboratory.
Commercial production commonly uses steam reforming and subsequent cryogenic liquefaction in plants run by Air Products and Chemicals, Inc. and Shell plc, with electrolytic hydrogen from Proton Exchange Membrane systems and Siemens electrolyzers increasing in relevance for low-carbon pathways promoted by International Energy Agency. Liquefaction requires multiple cooling stages and J-T or Claude cycle machinery developed with input from General Electric and Mitsubishi Heavy Industries. Long-term storage in Dewar flasks and vacuum-insulated cryogenic tanks incorporates multilayer insulation technologies created in collaboration with Boeing and Rolls-Royce Holdings for aerospace applications. Boil-off gas handling and reliquefaction systems are integral at terminals such as those planned by Hyundai Heavy Industries and at LNG terminals adapted by Kawasaki Heavy Industries. Liquid hydrogen's ortho-para conversion is catalyzed using materials studied at Stanford University and Caltech to manage heat release during storage.
Transport involves specialized ISO tank containers and road tankers certified under standards from International Organization for Standardization and regulations enforced by U.S. Department of Transportation and European Commission directives. Maritime carriage proposals reference standards from International Maritime Organization and projects by Hyundai Heavy Industries and Mitsubishi Shipbuilding. Ground handling for fueling stations developed with partners like Toyota and Hyundai Motor Company requires cryogenic couplings designed by engineering firms such as Parker Hannifin and safety systems modeled on protocols from National Fire Protection Association. Transfer operations use vacuum-jacketed piping and bayonet connections employed in facilities at Kennedy Space Center and Vandenberg Space Force Base, with boil-off management coordinated with vendors like Air Liquide and Linde plc.
Major applications include use as rocket propellant in cryogenic stages by programs like Ariane 5, Delta IV Heavy, and historical vehicles such as Space Shuttle main engines. Emerging uses feature fuel-cell vehicles promoted by Toyota Motor Corporation and Hyundai Motor Company and stationary power systems developed with support from Bloom Energy and Ballard Power Systems. Industrial feedstock roles involve hydrogenation and ammonia synthesis in plants run by Yara International and BASF SE, where liquid hydrogen enables high-purity applications at facilities managed by Ineos and Dow Chemical Company. Research applications at CERN, Lawrence Berkeley National Laboratory, and SLAC National Accelerator Laboratory include cryogenic cooling for superconducting magnets and detectors. Proposals for hydrogen-powered aviation by companies like ZeroAvia and Airbus explore liquid hydrogen for higher energy-density storage compared with compressed gas in long-range concepts.
Liquid hydrogen is flammable and presents distinct cryogenic hazards addressed in standards from Occupational Safety and Health Administration and National Institute for Occupational Safety and Health. Rapid vaporization can produce large vapor clouds with flammability risks analyzed in case studies by NASA and incident reports involving industrial sites overseen by U.S. Chemical Safety and Hazard Investigation Board. Materials embrittlement affects metals documented in research from Imperial College London and ETH Zurich, leading to selection guidelines used by Siemens and General Dynamics. Safety systems incorporate hydrogen sensors calibrated per methods from National Institute of Standards and Technology and automatic shutoff hardware specified by FM Global and Lloyd's Register. Emergency response planning references training programs at Federal Emergency Management Agency and industry consortia including Hydrogen Council partners.
Liquid hydrogen figures in low-carbon energy transitions promoted by Intergovernmental Panel on Climate Change scenarios and investment roadmaps from International Renewable Energy Agency and International Energy Agency. Environmental impacts depend on production pathway: electrolytic hydrogen powered by renewables as advocated by Ørsted and Vattenfall reduces lifecycle greenhouse gas emissions compared with steam-reformed hydrogen produced by companies like ExxonMobil and Chevron. Liquefaction energy intensity and boil-off losses influence cost analyses by consultancies such as McKinsey & Company and BloombergNEF, affecting competitiveness versus liquid fuels sold by BP and Shell plc. Infrastructure scaling involves public–private partnerships modeled on projects coordinated by European Commission programs and initiatives supported by United States Department of Energy and Japan's Ministry of Economy, Trade and Industry.