Characterization of heat and mass transport in magnesium hydride hydrogen storage systems
Hydrogen systems can provide viable alternatives for satisfying the world's energy requirements that both reduce dependency on and are more environmentally friendly than fossil fuels. For vehicular systems for example, hydrogen storage systems must have adequate gravimetric and volumetric storage capacities in addition to rapid uptake and release. Magnesium hydride shows good potential as a solid state hydrogen storage media due to its high gravimetric storage capacity of 7.6 wt% H2; the highest of all reversible metal hydrides. Drawbacks, however, are its large reaction enthalpy of 78 kJ/mol for the desorption reaction and its low thermal conductivity of ~1 W/m-K. In order to effectively transfer heat to and from an MgH2 hydride tank, both properties along with the thermal contact resistance at the tank boundary must be fully understood. In this work, the thermal properties of ball-milled and as-received MgH2 powder are characterized in hydrogen and inert environments over a range of temperatures, pressures, and reacted states using the transient plane source technique. The thermal conductivity of MgH2 was found to be slightly higher than that of Mg (0.7 W/m-K vs. 0.5 W/m K), whereas the diffusivity of MgH2 was found to be 2.5 times lower than that of Mg (0.62 mm2/s vs. 1.65 mm2/s). Increasing pressure resulted in an increase of thermal properties, while an increase in temperature resulted in a slight decrease of thermal properties. An optical reactor was designed and built to determine the effect of thermal contact resistance and to measure powder expansion of MgH2 during hydriding and dehydriding. Powders at porosities of 70% and 37% were measured in both absorbed desorbed states at temperatures ranging from 100°C to 300°C and pressures ranging from vacuum (10 -2 Torr) to 10 bar. A 2-D thermal model of the reactor was developed as a method of comparing experimental data to determine thermal contact resistance. Thermal contact resistance was found to be non-negligible and varied significantly with porosity. Powder packed at a porosity of 70% resulted in a thermal contact resistance of 10,000 mm2 K/W, whereas the contact resistance for powder packed at 37% porosity was only 100 mm2-K/W. Difference in thermal contact resistance between absorbed and desorbed powders was negligible, and no variation with either temperature or pressure was apparent. Thermal contact measurements were conducted with increased surface areas at the powder/vessel boundary of 100% and 200% for oxidized powder in atmospheric air. This increase in surface area was found to lead to an increase in thermal contact resistance for measurements at 200°C, but remained constant for measurements at 100°C. This optical reactor using MgH2 is the first of its kind and provides a possible test bed for other hydrides. It offers a range of capabilities, from the measurement of powder expansion to direct observation of heat and mass transport processes.
Pourpoint, Purdue University.
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