Modeling Thermochemical Energy Storage and Release in Redox Cycles of Ca1-XSrxMnO3-Δ
Authors: Imponenti Luca , Braun Robert J. , Jackson Gregory S. 
Published: February 2020

Effective thermochemical energy storage (TCES) at MWh or GWh scales through reduction of oxides requires materials with 1) thermodynamic potential for significant energy storage, 2) fast reversible surface chemistry for oxygen removal, 3) rapid oxide-ion diffusion through the bulk lattice, and 4) phase and structural integrity of high-surface area structures. As reported in our previous papers (1-4)and by others (5, 6) A-site doped Ca1-xSrxMnO3-δ (x <= 0.20) provides thermodynamics for significant reduction and reversibility during redox cycles for TCES at high temperatures ≥ 700ºC. With Sr-doping levels of 5 and 10%, Ca1-xSrxMnO3-δ has demonstrated thermodynamic limits for specific TCES of 664, 737, and 877 kJ/kg at 1000ºC at P
O2 ≈ 0.21, 0.01, and 10-4 bar respectively (from a zero reference state of fully oxidized Ca1-xSrxMnO3 in air at a cold temperature of 500ºC) (3). Ca0.90Sr0.10MnO3-δ and Ca0.95Sr0.05MnO3-δ were synthesized as porous particles through solid state reactive sintering to 1225ºC and tested for bulk reduction and re-oxidation at temperatures up to 1000ºC and P
O2down to »10-4bar to calibrate material models for predicting specific TCES in limited residence time reactors. The particles, illustrated in Figures 1, with diameters < 450 μm, porosity ≈ 0.6, and grain sizes from 1 to 10 mm were packed into annular beds and tested in isothermal and non-isothermal redox cycling between oxidizing and reducing gases.
Isothermal redox cycling of the porous particles in conjunction with TGA/DSC measurements provided the basis for fitting thermodynamically consistent surface reaction rate expressions and bulk ionic transport sub-models to measured evolution in δ. Figures 2 shows 10-cycle-averaged isothermal reduction and re-oxidation plots for a 0.6 cm bed of porous Ca0.9Sr0.1MnO3-δparticles cycled between P
O2 of 0.17 and 10-4 bar. Similar results between other P
O2 illustrate the significantly faster reoxidation rates that approach thermodynamic limits based on sweep-gas flow rates, suggest that neither bulk ionic diffusion nor O2 adsorption are rate-limiting at conditions tested.
The packed bed experiments were simulated using a two-level internal and external dusty-gas-model for porous media transport based on measured particle and bed properties. The results showed that for typical grain sizes of 5 mm, bulk-ion transport only impacted reduction rates at temperatures ≤ 700 ºC. As such, it was difficult to fit accurately ionic diffusion parameters and further testing is required to assess more accurately ambipolar bulk diffusion models for these materials. Fitted surface chemistry rates to the experimental measurements as shown in Figure 2 does capture both the slow reduction and the fast reoxidation with a thermodynamically consistent reversible rate expression adopted from previous studies (7).
The surface kinetics fitted for Ca0.9Sr0.1MnO3-δwere implemented in a one-dimensional indirect particle solar receiver model based on a narrow fluidized bed concept (8, 9)to explore the potential for how thermochemical energy storage in these particles could be coupled to designs and operating conditions for GWh energy storage systems for thermal power plants. The counterflow particles and fluidizing sweep gas at low P
O2 provides a basis for reduction but rapid buildup of O2 in the gas-phase near the bottom of the bed thermodynamically inhibits reduction. As shown in Figure 3, conditions that produce particle temperatures above 1000 ºC and reduction levels with δ > 0.1 and thus provide significant specific TCES. These conditions at relatively low solid flow rates result in high wall temperatures and thus lower solar efficiency (approaching 80%) due to the wall radiation losses. Material models for Ca1-xSrxMnO3-δ permit further exploration of receiver designs to find optimal geometries and operating conditions to achieve high δ at higher solar efficiency approaching targets of 90%.

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Figure 1