Search for: All records

Using hydrogen as a working fluid in cryocoolers can potentially benefit cryocooling technologies and hydrogen liquefaction. Moreover, in flow-through thermoacoustic systems, hydrogen can be efficiently cooled and undergo ortho-parahydrogen isomeric conversion, which is important for the efficient storage of cryogenic hydrogen. A traveling-wave regenerator is analyzed in this study, using the thermoacoustic theory with a superimposed mean flow and an empirical correlation for hydrogen isomer conversion. A regenerator with hydrogen fluid is shown to achieve higher performance in comparison with helium as the working fluid. However, the hydrogen system performance degrades at supercritical pressures and subcritical temperatures in compressed liquid states. In regenerators with mean flow, using hydrogen as the working fluid leads to higher cooling powers and efficiencies, but helium systems are able to achieve colder temperatures. 

Hydrogen represents a promising renewable fuel, and its broad application can lead to drastic reductions in greenhouse gas emissions. Keeping hydrogen in liquid form helps achieve high energy density, but also requires cryogenic conditions for storage as hydrogen evaporates at temperatures of about 20 K, which can lead to a large pressure build-up in the tank. This paper addresses the unsteady thermal modeling of cryogenic tanks with liquid hydrogen. Considering the liquid and vapor phases in the tank as two nodes with averaged properties, a lumped-element method of low computational cost is developed and used for simulating two regimes: self-pressurization (also known as autogenous pressurization, or pressure build-up in the closed tank due to external heat leaks) and constant-pressure venting (when some hydrogen is let out of the tank to maintain pressure at a fixed level). The model compares favorably (within several percent for pressure) to experimental observations for autogenous pressurization in a NASA liquid hydrogen tank. The two processes of interest in this study are numerically investigated in tanks of similar shapes but different sizes ranging from about 2 to 1200 m3. Pressure and temperature growth rates are characterized in closed tanks, where the interfacial mass transfer manifests initial condensation followed by more pronounced evaporation. In tanks where pressure is kept fixed by venting some hydrogen from the vapor domain of the tank, the initial venting rate significantly exceeds evaporation rate, but after a settling period, magnitudes of both rates approach each other and continue evolving at a slower pace. The largest tank demonstrates a six-times-lower pressure rise than the smallest tank over a 100 h period. The relative boil-off losses in continuously vented tanks are found to be approximately proportional to the inverse of the tank diameter, thus generally following simple Galilean scaling with a few percent deviation due to scale effects. The model developed in this work is flexible for analyzing a variety of processes in liquid hydrogen storage systems, raising efficiencies, which is critically important for a future economy based on renewable energy.