Exploitation of phase change materials for temperature control during the fast filling of hydrogen cylinders

This paper explores the use of phase change materials for the fast filling of hydrogen cylinders in order to limit the rise in the gas temperature by enhancing heat transfer from the gas. It is necessary to limit the temperature rise because the structural performance of the cylinder materials can be degraded at higher temperatures. Initially, two computational approaches for modelling the fast filling of hydrogen cylinders are presented and validated; the first is an axisymmetric computational fluid dynamics simulation and the second is a single-zone approach with one-dimensional conjugate heat transfer through the cylinder walls. The effect of incorporating paraffin wax-based phase change material within the cylinder structure has been investigated using the single-zone model. The predictions show that use of pure paraffin wax does not help to reduce the gas temperature due to its low thermal conductivity, however materials with improved thermal conductivity, for example mixtures of paraffin wax and graphite, can facilitate reduced fill times. The impact of phase change material is assessed in the case of a production hydrogen-powered passenger car. Without use of phase change material it is not possible to reduce the fill time below three minutes unless the gas supply is pre-cooled. While the fill time can be reduced by precooling the gas supply, the phase change material reduces the degree of pre-cooling required for a given fill time by 10-20 K, and reduces the minimum power consumption of the cooler by as much as 0.5% of the fuel’s calorific value

A model for the electrical conductivity of peak-aged and overaged Al-Zn-Mg-Cu alloys

A physically based model for the electrical conductivity of peak-aged and overaged Al-Zn-Mg-Cu (7xxx series) alloys is presented. The model includes calculations of the ?- and the S-phase solvus (using a regular-solution model), taking account of the capillary effect and ? coarsening. It takes account of the conductivity of grains (incorporating dissolved alloying elements, undissolved particles, and precipitates) and solute-depleted areas at the grain boundaries. Data from optical microscopy, differential scanning calorimetry (DSC), scanning electron microscopy (SEM) with energy-dispersive X-ray spectrometry (EDS), and transmission electron microscopy (TEM) are consistent with the model and its predictions. The model has been successfully used to fit and predict the conductivity data of a set of 7xxx alloys including both Zr-containing alloys and Cr-containing alloys under various aging conditions, achieving an accuracy of about 1 pct in predicting unseen conductivity data from this set of alloys