Thermodynamic modelling of low fill levels in cryogenic storage tanks for application to liquid hydrogen maritime transport

Abstract

One of the key challenges in transporting liquid hydrogen (LH2) in bulk onboard carriers is the requirement for high performance insulation to reduce the heat transfer and cargo boil-off losses. After unloading their cargo, carriers on the return voyage may retain a small amount of liquid (heel) to keep tanks relatively cold, as well as to chill down to the tanks prior to cargo loading. Therefore, there is an aim to reduce net heat transfer into the tank while minimising the required volume of heel. While practices are relatively mature in the liquefied natural gas (LNG) industry, it is not clear how this may differ for future LH2 carriage. In this study, a new analytical, lumped-mass model was proposed to predict vapour stratification and tank warm-up. A 40,000 m3 double-walled LH2 spherical tank was considered with vacuum perlite and polyurethane foam (PUF) insulation, and the effects of tank pressurisation and fill level on cumulative heat gain were investigated. The model pointed to key differences between the two insulation types. Heat gain in the vacuum perlite tank was not predicted to be sensitive to changes in fill level, while heat gain in PUF tank was predicted to be less sensitive to pressurisation. In each case, pressurisation was predicted to enable reductions in heel boil-off which offset any associated increase in cumulative heat gain. In comparing LNG and LH2, the model pointed to a slower thermal response of the LH2 tanks. Consequently, the increase in inner shell temperature was not expected to lead to significant reductions in environmental heat transfer into the tank. The findings of this study point to potential operational differences in the management of LNG and LH2 heel during the ballast voyage as well as considerations for vacuum insulated tanks.