(403a) Optimization of Hydrogen Storage Tanks: The Case of Sodium Alanate

Hydrogen is a very promising energy carrier for a comprehensive clean-energy concept in mobile applications. Regarding its use as fuel for the zero-emission vehicle, one main challenge is its storage. Hydrogen storage systems should fulfil the requirements of automotive applications, i.e. high gravimetric and volumetric storage densities, fast charging and discharging rates at moderate operating conditions, and high safety levels. Solid storage of hydrogen as metal hydrides offers a safe alternative to hydrogen storage in compressed or liquid form and has higher volumetric storage density. The light metal hydride sodium alanate, NaAlH4, compared to classical room temperature hydrides, offers a suitable compromise with relatively large gravimetric storage densities at rather moderate operating pressures and temperatures. Design of practical hydrogen storage systems aims at minimizing volume and weight while fulfilling defined criteria such as driving ranges and storage densities of the complete on-board tank system.

In the present investigation, hydrogen storage tanks based on sodium alanate are experimentally investigated, mathematically modelled, simulated and finally optimized. The determination of an optimal tubular storage tank based on sodium alanate material is presented. The optimisation is carried out on the basis of the predictions of a developed simulation tool for the hydrogen absorption, supported on the experimental results. Optimisation is performed for tubular tanks filled with loose powder and with compacted material. Compared to loose powder compacted material has the advantage of higher volumetric storage density and higher thermal conductivity, but the disadvantage of lower permeability to hydrogen flow and higher volumetric heat release during hydrogenation. Beyond compaction of the hydride material, the addition of expanded graphite is also evaluated in the optimisations. Although increasing the inert material of the system and thus reducing the total hydrogen storage capacity, the addition of expanded graphite enhances the effective thermal conductivity of the hydride bed.