Thermodynamically consistent modeling of redox-stable perovskite oxides for thermochemical energy conversion and storage

This paper describes an approach for thermodynamically consistent modeling of perovskite redox cycles for thermochemical energy storage and chemical-looping combustion applications. Prior modeling approaches to perovskite redox processes do not provide exact closure of the energy balance or thermodynamic consistency for calculating enthalpies and exergies of multiphase solid-gas flows needed in system-level process analysis. The approach documented here implements solid species thermodynamic functions derived from the enthalpies and entropies of reactions including oxidation/reduction and additional point defect reactions. The approach is fundamentally different than the typical approach of using partial molar properties to perform process flow modeling. Coupling process flow modeling to point defect reactions captures complex trends observed for oxygen non-stoichiometry and varying specific heat capacities during reduction and oxidation, maintains thermodynamic consistency between the solid and gas phase species, and thereby enables modeling of flow conservation equations for both the solid and gas phases. The thermodynamic model is fit to reported measurements for a highly reducible perovskite, strontium-doped calcium manganite (Ca0.6Sr0.4MnO3-delta), and then demonstrated through equilibrium thermodynamic calculations in process energy and exergy balances. Energetic and exergetic analyses for Ca0.6Sr0.4MnO3-delta redox cycles are presented for thermochemical energy storage and chemical-looping combustion systems where component exergy destructions are calculated. The model predicts roundtrip thermochemical energy storage efficiencies with Ca0.6Sr0.4MnO3-delta as high as 89% and 63% by first and second law analyses, respectively. The chemical-looping combustion of methane using Ca0.6Sr0.4MnO3-delta indicates first and second law efficiencies up to 90% and 66%, respectively. The modeling approach is used to explore trends in performance with operating conditions for both redox cycles thereby enabling new insight regarding design trade-offs for these emerging energy storage and conversion cycles. (C) 2015 Elsevier Ltd. All rights reserved.