(47c) Insulation System Design for Spherical Cryogenic Storage Tanks

The recent interest and efforts toward decarbonization have led to focus on developing the hydrogen economy. Hydrogen is a clean burning fuel and is the most abundant molecule in the universe. Moreover, hydrogen obtained from carbon neutral sources will be a big leap in decarbonization of the chemical industry as some of the highest carbon emitting processes (e.g. Ammonia) have hydrogen as their feedstock. Liquid form of hydrogen (LH2) has the highest gravimetric energy density, owing to which there have been pilot projects for transporting it through maritime route. LH2 is traditionally stored in spherical tanks with vacuum insulation. The vacuum technology is not scalable up to the commercial tank sizes (40,000-100,000 m³). Moreover, vacuum has several issues related to safety, durability and mechanical design which are exacerbated if the tanks are mounted on a ship. Therefore, it is prudent to develop a design that by-passes the vacuum. As a first step, non-vacuum technology needs accurate thermal modelling in order to predict the boil-off rate and prevent safety incidents like cryo-pumping, ice formation, etc. The intense temperature gradient of 20K to 300K across the insulation causes the physical properties of the gas filled in the insulation system to vary by an order of magnitude. We show that because the physical properties of gases (viscosity, density, and conductivity) have power law dependence on temperature, a temperature weighted stream-function allows for a neat analysis of the problem. Unlike horizontal geometry, where the bifurcation of convective solutions is of pitchfork type, the curvature effects in spherical systems lead to imperfect bifurcations with isolated multi-cell convective branches. We utilize spectral methods to construct the bifurcation diagrams and illustrate the regions of interest where multiple stable convective multi-cell solutions can co-exist with a crescent shaped weakly convective base solution.