Advancement of system designs and key engineering technologies for materials-based hydrogen storage

Experiments were performed in order to characterize the thermal integration aspects of 4 mol% TiCl3 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 degrees C, at which Na3AlH6 would no longer convert to NaAlH4 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. (C) 2013 United Technologies Corporation. Published by Elsevier B.V. All rights reserved.