Abstract
• SAH was shown to be a complex metal hydride material that compacts well by uniaxial pressing. • The cycling stability of SAH pellets was assessed and needs to be improved. • Compaction and certain additives significantly increase the thermal conductivity of SAH. • Optimizing the heat exchanger for a short refueling time can make the hydrogen adsorption kinetics of SAH rate limiting. Experiments were performed in order to characterize the thermal integration aspects of 4 mol% TiCl 3 doped sodium aluminum hydride (SAH) pellets with a heat exchanger tube. Models that were developed in previous work suggested that the rate of H 2 absorption could approach the kinetic limit of the SAH material when small SAH pellets would be integrated with relatively small diameter (1/8”) heat exchanger tubes. A method was developed to press SAH pellets with a center hole through which the heat exchanger tube could be inserted. Several pellets were stacked around a single heat exchanger tube and the test was instrumented with thermocouples in order to characterize the heat transfer process upon H 2 absorption. A comparison was made between SAH pellets with and without 5 wt.% Expanded Natural Graphite (ENG) worms for thermal conductivity enhancement. The results show a significant contact resistance between the SAH pellets and the heat exchanger tube (90–120 W/m 2 /K). ENG addition prevented the SAH pellets from reaching temperatures above 167 °C, at which Na 3 AlH 6 would no longer convert to NaAlH 4 at a H 2 pressure of 100 bar. ENG addition thereby reduced the refueling time from about 11 to 8 min. A further increase in the H 2 absorption kinetics appears to be required in order to reach DOE’s 2017 refueling time target of 3.3 min for 5.6 kg usable H 2 . The SAH pellets stacked around the heat exchanger tube expanded as a result of H 2 absorption and desorption cycles but their expansion was less than measured in the absence of the heat exchanger tube.
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