Technoeconomic investigation of optimal storage pressure and boil-off gas utilisation in large liquid hydrogen carriers

As countries transition away from carbon intensive energy sources, there may arise an opportunity to export carbon neutral fuels such as liquid hydrogen (LH2) to leverage the regional differences in renewable electricity costs. One of the challenges with the storage and transport of cryogenic liquids is the need to manage boil-off gas (BOG) due to heat ingress. Existing liquefied natural gas (LNG) carriers often utilise BOG as fuel for ship power, enabling savings in fuel purchase costs. Alternatively, allowing some pressure build-up in the tanks may serve to delay boil-off and reduce the quantity of cargo lost during the voyage. This study investigated the effect of varying fuel type, ship speed and tank maximum allowable working pressure (MAWP) on the net present value (NPV) of a 160,000 m3 LH2 carrier. A combined gas and steam (COGAS) engine was considered, utilising either marine gas oil (MGO) or hydrogen. A lumped mass analytical model was used to estimate the storage boil-off, while analytical speed-resistance correlations were used to estimate ship fuel consumption. The model pointed to a preference toward reducing cruising speed when operating along greater voyage distances. The preference toward either marine gas oil (MGO) or hydrogen for power was highly dependent on carbon pricing. Without carbon pricing however, using conventional fuel was consistently preferable to hydrogen when a high tank MAWP was selected. When using hydrogen for fuel, generally no benefit was observed when operating storage tanks above atmospheric pressure due to ability to utilise any vapour vented. Overall, optimising both speed and tank MAWP was observed to increase net present value by up to $180 M. However, sensitivity analysis suggested that external factors, such as voyage distance and levelised cost of production, remained the primary determinants of economic feasibility.