Local Area Cooling versus Broad Area Cooling for Boil-Off Reduction in Large-Scale Liquid Hydrogen Storage Tanks

Future use of liquid hydrogen (LH$_2$) as an effective energy carrier will require elimination or minimization of hydrogen boil-off that is not utilized by demands in the value chain. The present work promotes local area cooling (LAC) as a promising boil-off reduction technology. In contrast to the more conventional broad area cooling (BAC), LAC targets local, concentrated heat flows e.g. through tank support structures. This yields important practical benefits, especially for large-scale tanks, due to the order-of-magnitude reduction in the size of the cooling system. Such benefits include lower capital costs and simpler installation, maintenance and coolant management. LAC applied outside the outer tank wall is particularly attractive for tanks with evacuated insulation. In a series of numerical studies, we use the finite element method to evaluate the thermal performance of LAC and BAC in the context of ship-borne LH$_2$ transport. The studies concern 40 000 m$^3$-capacity, skirt-supported tanks insulated using evacuated perlite or helium-filled polyurethane (HePUR) foam. For the perlite-insulated tank, LAC and BAC with liquid nitrogen coolant can reduce the daily boiloff rate from 0.04%/day to, respectively, 0.011%/day and 0.004%/day. The corresponding numbers for CO$_2$-based refrigeration are 0.031%/day and 0.028%/day. For the HePUR-insulated tank, which has a higher baseline boil-off rate of 0.24%/day, reduced boil-off rates down to 0.17%/day and 0.04%/day are achievable using LAC and BAC, respectively. LAC and BAC both offer increased power efficiency in comparison to reliquefaction only.