Optimized para fraction of liquid hydrogen for efficient liquefaction and storage

Abstract A thermodynamic study is performed to determine the optimal para fraction of liquid hydrogen for efficient liquefaction and storage. Ortho-to-para (OP) conversion is required in liquefaction process to avoid the boil-off loss during liquid storage. In case of short-term storage, however, the liquefaction without conversion could be advantageous, since the natural conversion is a slow process and a liquefier is capable of producing more liquid without catalytic OP conversion. A rigorous cycle analysis is carried out to predict the liquefaction capacity of a small-scale liquefier, where four different levels of para fraction can be obtained by the optional use of staged catalytic conversion. By incorporating a kinetic model for natural OP conversion in a storage tank, it is revealed that there exists an optimal para fraction for efficient liquefaction and storage, depending on the length of storage period. No catalytic conversion is recommended for the storage of 1 day or less, but the full conversion is needed for storage over 5 days.