The favourable thermodynamic properties of Fe-doped CaMnO3for thermochemical heat storage

Abstract

The CaMnO3oxide can reversibly release oxygen over a relatively wide range of temperatures and oxygen partial pressures (pO2) and is thus a promising candidate for thermochemical heat storage in Concentrated Solar Power (CSP) plants. Moreover, it is composed of earth-abundant, inexpensive and non-toxic elements and exhibits a high-energy storage density, which are desirable characteristics for decreasing the deployment costs of the system. However, it undergoes decomposition atpO2≤ 0.008 atm and temperature ≥ 1100 °C. Here the possibility of overcoming this limitation and extending the operating temperature range by B-site doping with Fe (CaFexMn1−xO3−δ0) is explored. Two doping levels are investigated,x= 0.1 and 0.3. The enthalpy of reduction was determined from a measurement of continuous equilibrium non-stoichiometry curves (δ,T) at severalpO2, enabling an evaluation of the heat storage capacity with high accuracy over widely ranging oxygen non-stoichiometry. Introduction of 0.1 Fe (CaFe0.1Mn0.9O3−δ0) prevented CaMnO3decomposition up to 1200 °C atpO2= 0.008 atm, thus widening the operating temperature range and the oxygen reduction extent. The increase in the accessible nonstoichiometry translates into an increase in the heat storage capacity (QM(kJ molABO3−1)) from ∼272 kJ kgABO3−1in CaMnO3to ∼344 kJ kgABO3−1in CaFe0.1Mn0.9O3−δ0.While even larger changes in oxygen content were accessible in CaFe0.3Mn0.7O3−δ0, the oxidation state changes are accompanied by a lower enthalpy of reduction, resulting in a diminished heat storage capacity of ∼221 kJ kgABO3−1