Modelling of metal hydride hydrogen compressors from thermodynamics of hydrogen - Metal interactions viewpoint: Part I. Assessment of the performance of metal hydride materials

This work presents a model to determine productivity and heat consumption of hydrogen compression utilising metal hydrides (MH) by using Pressure - Composition - Temperature (PCT) diagrams of the MH materials at defined operating conditions - temperatures and hydrogen pressures. The present Part I is focused on the analysis of hydrogen compression performances of several AB(5)- and AB(2)-type intermetallic alloys which, when operating between temperatures of 20 and 150 degrees C, provide H-2 compression up to 500 atm, with a cycle productivity about 100 NL H-2/kg MH and compression ratio of up to 10, at H-2 suction pressure below 10-15 atm, or up to 5 at higher suction pressures. We show that calculated cycle productivities of hydrogen compression are related to the operating conditions and significantly vary for the different MH materials, even though showing similar trends in their changes. The cycle productivity of MH material increases with decrease of the cooling temperature, decrease of the discharge pressure, increase of the heating temperature and increase of the suction pressure. When hydrogen pressure approaches plateau pressures for H-2 absorption at cooling or H-2 desorption at heating, the changes of the cycle productivity become very pronounced. Particularly, the compression productivity becomes very sensitive to the P-T variations when the isotherms show presence of "flat" pressure plateaux which are characteristic for the ideal PCT diagrams of the MH. Thus, in the latter case, even minor changes in P-T result in a dramatic variation of the cycle productivity and when aiming at increased efficiency of the process, a strict P-T control is required. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.