Abstract
Hydrogen is envisioned to play a significant role in the transition away from fossil fuels and towards net-zero emissions. In this context, liquid hydrogen (LH2) is a relevant alternative for large-scale hydrogen transport and storage. Drawing inspiration from existing liquid natural gas (LNG) carrier ships, there is significant industrial, academic and political interest in realizing LH2 carrier ships with a capacity of about 100 000 m3 or more. A key technical challenge in this endeavor is to limit evaporation of the LH2, which is stored at −253 ◦C. Accurate thermal modeling is thus paramount to the successful assessment of LH2 carrier ships.
In broad terms, this thesis contributes numerical simulation methods and results relevant to the evaluation and development of large-scale (40 000 m3) LH2 carrier storage tanks. To account for the unique challenges of ship-borne transport, the conceptual designs considered herein are of skirt-supported tanks, similar to existing ship-borne LNG tanks of the Moss type. The presented simulation methods were largely based on the finite element method, which enables efficient representation of complex tank geometries, and implemented using the open-source finite element library Netgen/NGSolve. These methods enable accurate, high- fidelity modeling of steady-state and transient heat transfer in tanks with bulk fill insulation, steady-state heat transfer in tanks with multilayer insulation (MLI), and heat transfer in tanks with stratified content.
Using the presented methods, the following aspects of large-scale, skirt- supported, ship-borne LH2 tanks were investigated: 1) Boil-off rates and temperature fields in MLI-insulated and bulk fill-insulated tanks, 2) The impact of active cooling solutions on boil-off, 3) Power consumption of active cooling solutions in comparison to that of reliquefaction, 4) Boil- off rate sensitivity to tank design choices and boundary temperatures. Additionally, the methods were used to validate more rudimentary but faster thermal network models, and to study heat transfer paths in an LH2 tank with stratified content.
Key practical takeaways from the present work include: 1) Boil-off rates of around 0.01 %/day and 0.04 %/day appear achievable for large-scale, ship-borne LH2 tanks with and without active cooling, respectively, 2) Local cooling of the support structure appears attractive due to reduced evaporation of LH2 and reduced power consumption in comparison to reliquefaction, 3) It is beneficial to apply MLI to the support structures of MLI-insulated tanks, 4) Heat ingress and boil-off are generally more sensitive to the ambient temperature than to the temperature inside the tank, 5) The presented simulation methods are general in nature and can be adapted to study other LH2 tanks or cold/cryogenic systems through relatively minor modifications. This broad applicability has been demonstrated in a study concerning rapid depressurization of a CO2 tank.
Finally, the thesis includes a review article that covers key performance trends and knowledge gaps in the open MLI literature. The parameter sensitivities of the established layer-by-layer method are explored with emphasis on the predicted cross-over pressure, i.e., the pressure where gas conduction through the interstitial gas becomes significant. It was further revealed that, although interstitial gas pressure is widely accepted as decisively important for MLI performance, the practically achievable pressures and evacuation times reported in the literature vary by orders of magnitude. This is a significant source of uncertainty when evaluating MLI’s suitability for novel cryogenic systems, such as large-scale LH2 tanks, and warrants further investigation. Other suggested avenues for future research on MLI performance include gas sorption, freeze-out (particularly during tank cool-down and loss-of-vacuum events), MLI performance at medium-range pressures, and heat transfer through MLI applied to non-isothermal surfaces.