Performance investigation of metal hydride reactors adopting multilayer bed with graded content of expanded natural graphite for thermochemical heat storage

Abstract

Metal hydrides (MHs) are promising materials for thermochemical heat storage. However, poor heat transfer limits the hydrogen absorption/desorption processes in MH reactors. In this study, a modified multilayer MH bed configuration with graded content of expanded natural graphite (ENG) was proposed. A mathematical model was established to obtain the distributions of temperature and reacted fraction within the MH beds during hydrogen absorption. Two comprehensive indices, gravimetric heat storage rate (GHSR) and gravimetric exergy-output rate (GEOR), were adopted to evaluate the heat discharging performance of the MH reactors. The simulation results show that a configuration in which the direction of the ENG content gradient (β) is consistent with the direction of heat flow can improve the performance of the MH reactors. Furthermore, increasing steepness of the ENG content gradient can increase both of the GHSR and the GEOR. The configuration with the squared graded ENG contents (C5-7, β = 3.5 wt%) exhibits the highest GHSR and GEOR values (16.98 and 16.16 W kg−1, respectively), whereas the configuration C1 which employs a uniform ENG content exhibits much lower values of these parameters.