Modeling the effects of liquid hydrogen tank operations on boil-off losses

Abstract

Popular choices for storing bulk hydrogen include 100 bar gas, 200 bar gas (GH2), and liquid (LH2) kept in cryogenic tanks. LH2 achieves a greater volumetric density than GH2 albeit with ambient heat load causing hydrogen boil-off and self-pressurization. Although many models are available in the literature for self-pressurization, few if any studies have applied these models to real-world tank operational scenarios, such as filling or venting, to determine optimal tank settings. To address this need, a non-thermal equilibrium reduced-order model is developed and validated using experimental data from NASA's Integrated Refrigeration and Storage System (IRAS) and an in-service LH2 storage tank in industry. Five parametric studies are conducted to explore the effects of liquid extraction with initial tank fill level, tank filling conditions, vapor return percentage, temperature of vapor return, and saturated liquid return on boil-off losses. One parametric study estimates a boil-off reduction of 15 % by extracting hydrogen from the tank as a liquid with 30 % vapor return compared to a cyclic venting baseline study. Results indicate that operational modes for LH2 storage tanks have a significant impact on boil-off losses relative to initial tank design considerations and should be considered in the implementation of LH2 storage tanks.