Thermochemical energy storage in strontium-doped calcium manganites for concentrating solar power applications

Abstract

Thermochemical energy storage (TCES) using redox cycles of reducible perovskite oxides can potentially provide higher specific energy capacities and storage temperatures than molten-salt systems for large-scale energy storage in concentrating solar power (CSP) and other applications. Perovskites from earth abundant cations, such as CaMnO3, are needed for cost-effective solutions, but such materials must demonstrate favorable thermodynamics for high specific TCES (chemical+sensible) and favorable kinetics for heat-driven reduction and exothermic re-oxidation in the redox cycle. This paper explores the thermodynamics and kinetics of Ca1-xSrxMnO3-δ(x=0.05and0.1) particles for TCES redox cycles where the particles are heated and reduced in N2 (PO2≈10-4bar) to high temperatures TH up to 1000°C in a solid-particle solar receiver. Chemical and sensible energy stored in the reduced perovskite particles is released as needed to a power cycle via re-oxidation and cooling of the material. Variation of oxygen non-stoichiometry (δ) of Ca1-xSrxMnO3-δ with temperature and PO2 is measured via thermogravimetric analysis. Reaction enthalpy for reduction and re-oxidation is determined by fitting the variation of δ with temperature and PO2 to a two-reaction point defect model. The fits compare favorably to differential scanning calorimetry with overall reaction enthalpies varying significantly with δ. For CSP applications, limited time in the solar receiver for perovskite reduction requires fast kinetics to achieve high specific TCES. Once the material is characterized thermodynamically, kinetic measurements for particle reduction and re-oxidation are performed in a packed-bed reactor to assess rates of O2 release and uptake at various temperatures and PO2 with particles of diameter between250and425μm. Packed-bed experiments indicate that the low values of A-site Sr-doping stabilize the CaMnO3-δ structure and allow large δ at high temperatures and PO2≈10-4bar. In these time-limited redox cycling experiments between 500and900°C, mass-transfer limited kinetics allows the Ca0.95Sr0.05MnO3-δ to reach a specific TCES of 620kJkg−1, which is 78% of the equilibrium limit of 791kJkg−1. X-ray diffraction after 1000 redox cycles showed that the Ca1-xSrxMnO3-δ particles have excellent phase stability for the desired redox conditions.