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

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-delta (x = 0.05 and 0.1) particles for TCES redox cycles where the particles are heated and reduced in N-2 (P-O2 approximate to 10(-4) bar) to high temperatures T-H up to 1000 degrees 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 (delta) of Ca1-xSrxMnO3-delta with temperature and P-O2 is measured via thermogravimetric analysis. Reaction enthalpy for reduction and re -oxidation is determined by fitting the variation of delta with temperature and P-O2 to a two-reaction point defect model. The fits compare favorably to differential scanning calorimetry with overall reaction enthalpies varying significantly with delta. 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 O-2 release and uptake at various temperatures and P-O2 with particles of diameter between250 and 425 mu m. Packed-bed experiments indicate that the low values of A -site Sr-doping stabilize the CaMnO3-delta structure and allow large delta at high temperatures and P-O2 approximate to 10(-4) bar. In these time -limited redox cycling experiments between 500 and 900 degrees C, mass-transfer limited kinetics allows the Ca(0.95)Sr(0.05)MnO(3-delta)to reach a specific TCES of 620 kJ kg(-1), which is 78% of the equilibrium limit of 791 kJ kg(-1). X-ray diffraction after 1000 redox cycles showed that the Ca1-xSrxMnO3-delta particles have excellent phase stability for the desired redox conditions. (C) 2017 Elsevier Ltd. All rights reserved.