Our team has explored redox cycles of doped CaMnO3-δ between air and low O2 partial pressures (~10-4 bar) for high-temperature thermochemical energy storage (TCES) applications. In this study, we have explored both A-site and B-site doped compositions using earth-abundant cations to identify perovskites for cost-effective TCES in concentrated solar and other high-temperature, thermal storage applications. Reduction of doped CaMnO3-δ above 800 °C in the low P O2 requires some amount of dopant to avoid irreversible decomposition of the perovskite structure observed for reduction of undoped CaMnO3-δ [1]. This study shows that small amounts (5%) of A-site or B-site dopant can stabilize the CaMnO3-δ perovskite structure during reduction at temperatures up to 1100 °C and P O2 = 10-4 bar. For selected, stable doped CaMnO3-δ compositions, total specific TCES (Δh tot) was defined by the sum of specific sensible energy (Δh sens) and chemical energy (Δh chem) captured during heating and reduction from air at a cool temperature (T C fixed at 500 °C in this study) to low P O2 at varying high temperatures ( T H). B-site doping with 5% Cr and A-site doping with 5% Sr provided thermodynamic limits of Δh tot over 720 kJ kg-1 and 790 kJ kg-1 respectively for T H = 1000 °C. These materials were also tested kientically to assess the rates at which energy storage is captured during reduction and released during oxidation as relevant for concentrated solar energy storage applications. The results indicate that the doped CaMnO3-δ and in particular A-site doped Ca0.95Sr0.05MnO3-δ have promise as a TCES storage media with Δh chem providing just under half of the total energy stored during the combined heating and reduction. For the perovskite redox cycles, the heat of oxide reduction -ΔH O can vary with non-stoichometry δ [2]. To find the integrated Δh chem, the functional dependence of ΔH O with δ must be detemined. Two methods have been undertaken to explore this functional dependence: 1) combined TGA-DSC calorimetry measurements with incremental changes in δ, and 2) point-defect model fitting to equilibrium δ vs. P O2 data from TGA measurements [3,4]. The DSC measurements showed that the magnitude of ΔH O decreases to a near constant value for δ > 0.1 as illustrated for Ca0.95Sr0.05MnO3-δ in Figure 1. Similar trends were observed for the other perovskite compositions. The alternative method for finding ΔH O as a function of δ by fitting the equilibrium δ vs. P O2 involved modeling the perovskite reduction with two reversible point-defect reactions -- oxide-ion vacancy formation and Mn cation disproportionation from 4+ to 3+ and 5+ states [4]. The model fits originally assumed that these two reactions had enthalpies independent of δ but the contribution of the disproportionation reaction increased with δ thereby causing the combined ΔH O to have a minor dependence on δ as also shown in Figure 1. The point defect modeling did not show the same trends for ΔH O vs. δ but provided very similar values at high δ > 0.1. The integration of ΔH O over a change in δ (i.e. Δδ as in Table 1) provides a basis for calculating the integrated Δh chem as a function of T H. Integrated Δh chem values for CaCr0.05Mn0.95O3-δ, Ca0.95Sr0.05MnO3-δ, and Ca0.9Sr0.1MnO3-δ are shown in Table 1 along with the total energy stored which includes Δh sens derived from integration of CP with respect to T over the redox cycle heating. The higher Δh chem and associated Δh tot for Ca0.95Sr0.05MnO3-δ stems from its increase reducibility without a loss in heat of reaction and makes it a more promising material for TCES applications in these temperature ranges.For thermal storage application, kinetic rates of TCES capture are important, particularly for concentrated solar applications where solar receiver residence times are limited. To explore kinetics, reduction and oxidation rates of porous perovskite particle beds were measured for fitting thermodynamically consistent kinetic models to the observed rates. Kinetic testing and model fitting for the Ca0.95Sr0.05MnO3-δ indicates that it achieves approximately 80% of its Δh chem thermodynamic limit in 60 s exposure of low P O2 gas for reduction at T H = 900 °C. On the other hand, at the same temperature reoxidation occurs rapidly and releases over 90% of the Δh chem limit in 30 s. The thermodynamic consistent model involving surface kinetics and ionic bulk diffusion in the perovskite particles for Ca0.95Sr0.05MnO3-δ and for CaCr0.05Mn0.95O3-δ provide a basis for designing reactors for redox cycles to be used in TCES for concentrated solar and other potential thermal energy storage applications.
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Sep 3, 2024
Chiemeka Onyeka Okoye + 9
Open Access