Para-H2 to ortho-H2 conversion in a full-scale automotive cryogenic pressurized hydrogen storage up to 345 bar

• Natural para-H2 to ortho-H2 conversion was studied in a full scale (151 L) cryogenic pressure vessel.
• Relative mixture was measured by Raman spectroscopy and calorimetry.
• H2 density was varied between 14 and 67 g/L, and pressure from atmospheric to 345 bar.
• Discrepancies with literature were observed mainly at low density and ortho fraction.
• 3–7 days dormancy enhancements could be attributed to para–ortho conversion.

Hydrogen vehicles offer the potential to improve energy independence and lower emissions but suffer from reduced driving range. Cryogenic pressure vessel storage (also known as cryo-compressed storage) offers the advantage of higher densities than room temperature compressed although it has the disadvantage of cryogenic operating temperatures which results in boil-off when the temperature of the gas increases. In order to understand and optimize the time prior to boil-off, we have examined heat absorption from the transition between the two quantum states of the hydrogen molecule (para–ortho) in a full-scale (151 L internal volume) automotive cryogenic pressure vessel at pressures and temperatures up to 345 bar and 300 K, and densities between 14 and 67 g/L (2.1–10.1 kg H2). The relative concentration of the two species was measured using rotational Raman scattering and verified by calorimetry. In fifteen experiments spanning a full year, we repeatedly filled the vessel with saturated LH2 at near ambient pressure (2–3 bar), very low temperatures (20.3–25 K), varying densities, and very high para-H2 fraction (99.7%). We subsequently monitored vessel pressure and temperature while performing periodic ortho-H2 concentration measurements with rotational Raman scattering as the vessel warmed up and pressurized due to environmental heat entry. Experiments show that para–ortho H2 conversion typically becomes active after 10–15 days of dormancy (“initiation” stage), when H2 temperature reaches 70–80 K. Para–ortho H2 conversion then approaches completion (equilibrium) in 25–30 days, when the vessel reaches 100–120 K at ∼50 g/L density. Warmer temperatures are necessary for conversion at lower densities, but the number of days remains unchanged. Vessel dormancy (time that the vessel can absorb heat from the environment before having to vent fuel to avoid exceeding vessel rating) increased between 3 and 7 days depending on hydrogen density, therefore indicating a potentially large benefit for reduced fuel venting in cryogenic pressurized hydrogen storage.