Alloy selection for multistage metal-hydride hydrogen compressors: A thermodynamic model

A thermodynamic model is presented to aid the selection of compatible pairs of hydrogen storage alloys for service in a multi-stage metal-hydride compressor. The model is built around the concept of an ideal compressor in which all pairs of hydrides operate between the same working temperatures. The key feature of the model is a link between the thermodynamic characteristics of the hydrides in each pair (the entropy change ΔS and enthalpy change ΔH when the hydride is formed from the metal) that results from requiring that the heated low-pressure hydride is able to desorb to the cooled high-pressure metal at a unique intermediate pressure. This necessary linking places severe constraints on the choice of alloys, since ΔS and ΔH cannot be freely chosen and are in fact strongly correlated for hydrides with similar operating temperatures. The model is based on the van ‘t Hoff relation, which is derived from first principles and shown to be only approximately valid at the high hydrogen pressures of interest for vehicle filling stations. Pressure hysteresis and plateau slope are incorporated into the model. A case study of a two-stage compressor is presented, based on the curved van ‘t Hoff line and the experimentally determined effects of pressure hysteresis and plateau slope.