Optimization of hydrogen charging process parameters for an advanced complex hydride reactor concept

Abstract

Complex hydrides are identified as promising hydrogen storage media with gravimetric capacities up to 10 wt.%. However, the high temperatures required for the initiation of their hydrogen charging process and their slow kinetics prevent their integration in many practical applications. This paper discusses the challenge of addressing these issues by combining this kind of materials with the appropriate metal hydrides. For this purpose, the complex hydride, 2LiNH2–1.1MgH2–0.1LiBH4–3 wt.% ZrCoH3 (CxH) and the metal hydride, LaNi4.3Al0.4Mn0.3 (MeH) have been selected as reference materials. The studied configuration corresponds to a tubular reactor where the metal hydride and the complex hydride, separated by a gas permeable layer, are embedded respectively in the centre and the annular ring of the reactor. A 1-dimensional finite element model and a dimensionless number comparing the dominance of the kinetics and the heat transfer processes have been developed to optimize the charging process for different thicknesses and volumetric ratios of the studied materials. For the selected cases, the influence of the thermal properties of the complex hydride and the operating conditions on the charging process is assessed. A sensitivity study has shown that the thermal conductivity of the CxH is the most important parameter influencing the hydrogen storage rate if thick MeH and CxH beds are considered. In contrast, the hydrogen loading time is significantly improved by increasing the coolant temperature for small thicknesses of the two storage media. Thereafter, the gravimetric and volumetric capacities resulting from the scale up of the optimized configurations to store 1 kg of hydrogen are calculated and results are discussed taking into account the interdependence of the different studied parameters.