Thermal performance of cylindrical and spherical liquid hydrogen tanks

In liquid hydrogen (LH2) storage tanks, the temperature difference between LH2 and the environment leads to the inevitable heat ingress into the storage tanks. Understanding the details of the thermal performance in LH2 storage tanks is necessary to minimize or avoid boil-off gas losses. This paper presents a thermodynamic nonequilibrium model to study the thermal differences between cylindrical and spherical LH2 tanks with the same volumetric capacity. The accuracy of the model is validated with experimental data obtained from cylindrical and spherical tanks. The numerical results show that, in self-pressurized tanks, a spherical geometry results in the lowest pressure increase rate. The pressure increase in horizontal, cylindrical tanks is slower than the corresponding increase in cylindrical tanks oriented vertically. The difference can be related to more effective heat transfer between the vapor and liquid phases in the horizontal cylindrical tanks. A higher initial liquid level increases the thermal mass of the liquid, which suppresses the evaporation rates due to larger heat requirements to warm and evaporate the liquid hydrogen. For tanks under atmospheric pressure, the boil-off gas rate is lowest in the spherical tank while the boil-off gas rates in the horizontal and vertical cylindrical tanks are almost the same. Under these conditions, and quasi-steady state, the total heat absorbed by the tanks is used to evaporate LH2. The spherical tank has the smallest surface area and thus the lowest evaporation rate and the lowest heat transfer from the environment. The evaporation rate is not affected by initial liquid levels.