Liquid Hydrogen

Hydrogen becomes a liquid at 20K and 1 bar. Liquid hydrogen (LH₂) is an important substance in industry, transportation and research. Industrial uses of hydrogen include applications in the electronics, glass, chemical and metal processing fields. In these applications, the hydrogen is generally used as a room temperature gas but is frequently transported and stored as a liquid to save space. The principal use of hydrogen in transportation is as a fuel for space propulsion. In this application, the hydrogen is always transported, stored and used as a liquid. A growing application of hydrogen is its use as an automobile fuel either in internal combustion engines or fuel cells. Here again, the end use of the hydrogen may be as a room temperature gas but its storage and transport frequently occurs in the liquid state. Transportation and storage of liquid hydrogen will be a significant part of any future economy based on hydrogen fuels.

Hydrogen is used almost exclusively in liquid form in scientific research applications. During the 1960s, 1970s and into the early 1980s, liquid hydrogen bubble chambers were used to detect and identify subatomic particles in high energy physics labs like Fermilab and CERN. While this technique has been replaced by other detector designs, hydrogen still plays an important role in research as a mechanism for slowing down neutrons (moderators) or charged particle beams (absorbers) and as a target material for electron/electron or electron/proton scattering experiments. In all these cases, hydrogen is used in liquid form, as the increased density of the liquid increases the number of interactions between the hydrogen and the particles.

Hydrogen occurs in two forms, based on the orientation of the nuclear spins of the two protons in the diatomic hydrogen molecule. Orthohydrogen occurs when these spins are parallel and parahydrogen occurs when these spins are antiparallel. Para and ortho hydrogen have different thermodynamic properties, and care must be taken to understand the composition of any hydrogen in use. At room temperature the equilibrium composition of hydrogen is 25 percent para and 75 percent ortho. At 20K, the equilibrium concentration is 99.8 percent para. When hydrogen is liquefied, it will naturally convert from the ortho to para forms. However, this conversion is slow and exothermic, releasing heat that will boil off the hydrogen. In order to avoid this problem, hydrogen liquefiers generally contain a catalyst, such as nickel silicate, to speed the conversion to the para form.

Hydrogen has a very wide flammability range and liquid hydrogen systems must be designed with carefully thought out safety systems that eliminate the unintentional mixing of the hydrogen with air or oxygen and that allow safe venting.

A good overview of the uses of liquid hydrogen can be found in “The Commercial Uses of Liquid Hydrogen over the Last 40 Years,” G.E Kinard, Proc. ICEC 17 (1998). A recent example of the aerospace uses of liquid hydrogen is “Design of a Reconfigurable Liquid Hydrogen Fuel Tank for use in the Genii Unmanned Aerial Vehicle,” P. Adam et a.l., Adv. Cryo. Engr. Vol 59B (2014). Examples of the use of liquid hydrogen in moderators, absorbers and targets are given in “A Liquid Cryogen Absorber for MICE,” D.D. Baynham et al., Adv. Cryo. Engr. Vol 51B (2006), ”Liquid Hydrogen Target for the COMPASS Experiment,” J. Bremer et al., Adv. Cryo. Engr. Vol 59A (2014), “Operational Characyeristics of the J-PARC Cryogenic Hydrogen System for a Spallation Neutron Source,” H. Tatsumoto et al. Adv. Cryo. Engr. Vol 59A (2014) and “Challenges and Design Solutions of the Liquid Hydrogen Circuit at the European Spallation Source,” S. Gallimore et al., Adv. Cryo. Engr. Vol 59A (2014).

Innovation continues in the efficient liquefaction of hydrogen. Two recent examples are “Investigation on the Two-Stage Active Magnetic Regenerative Refrigerator for Liquefaction of Hydrogen,” I. Park, et al. and “Selection of Components for the IDEALHY Preferred Cycle for the Large Scale Liquefaction of Hydrogen,” H. Quack et al., both in Adv. Cryo. Engr. Vol 59A (2014).