Sodium alanate hydrogen storage system for automotive fuel cells

Suitability of Ti-catalyzed sodium alanate as a reversible hydrogen storage system for automotive polymer–electrolyte fuel cells has been assessed under the constraint that it must meet certain performance criteria with respect to the amount of recoverable H2H2 needed for vehicle driving range, minimum full-flow of H2H2 needed to satisfy vehicular power demands, maximum refueling time and minimum H2H2 delivery pressure. It is assumed that the thermal energy for desorbing hydrogen is supplied by the fluid that cools the high-temperature membrane fuel cell stack operating at 120∘C. It is found that a ten-fold enhancement in published desorption kinetics is needed to attain 90% reversible H2H2 storage capacity while satisfying the minimum full-flow requirement of 0.02 g H2H2/kWe. The catalyzed medium needs a metal foam support to facilitate heat removal necessary for 0.5–1.5 kg/min H2H2 refueling rate. The faster the refueling rate, the higher is the peak heat transfer rate and the lower is the amount of recoverable H2H2. For a system with 5.6 kg recoverable H2H2, the peak heat transfer rate during refueling can exceed 1 MW. The overall specific energy and energy density of the storage medium are functions of the packing density of the metal hydride powder. The minimum acceptable hydrogen delivery pressure from the storage device determines the hydrogen storage capacity but otherwise has small influence on sorption kinetics.