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

摘要

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_3AlH_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.