A holistic approach for the development of CaMnO -based perovskite materials for hybrid sensible-thermochemical storage in next generation Concentrating Solar Thermal technologies

Kurzfassung

Ca-Mn-based perovskites have been acknowledged as attractive materials for concentrated solar thermal (CST) heat storage via a hybrid sensible-thermochemical mode. Their suitability is attributed to their capability for cyclic reduction-oxidation (redox) under air atmosphere accompanied by significant endothermal/exothermal heat effects and complete reversibility of oxygen uptake/release and dimensional changes due to thermochemical expansion/contraction. Of equal importance are also criteria referring to the low cost, earth abundance and environmentally benign character of their constituting elements. However, for their eventual large-scale exploitation in next generation CST plants targeting high-temperature processes, properties relevant to structural integrity and resilience of the perovskite structured shapes (i.e. monolithic porous bodies, granules or pellets) to be eventually used therein have also to be taken into consideration and optimized. The present work addresses the development of CaMnO3-based optimized compositions in a holistic approach, starting from high-throughput computational screening of such A- and B-site doped compositions to extract via Density Functional Theory (DFT) theoretical key metrics like heat capacity (extremely important for sensible heat storage), redox reactions enthalpy and thermo-mechanical properties within the temperature range of interest, as a function of kind and concentration of dopant elements. In the next step, the identified shortlisted multi-cation perovskite compositions were synthesized by solid-state and liquid-phase routes, optimizing selection of precursor powders and sintering conditions with respect to the phase purity of the obtained perovskite. The synthesized powders were characterized with respect to physicochemical properties like specific surface area, phase stability and thermochemical expansion/contraction by nitrogen porosimetry, thermogravimetry/ differential scanning calorimetry (TGA/DSC) and dilatometry respectively. In-situ high-temperature x-ray diffraction (XRD) was also employed under a wide range of temperature and oxygen partial pressures, to correlate such characteristics to possible phase transformations and dimensional changes. Key properties of merit relevant to the targeted hybrid sensible-thermochemical heat storage application, namely heat capacity, extend of reduction and heat effects of the redox reactions were experimentally determined in the temperature range 300-1100oC under varying oxygen partial pressure. It was shown that suitably doped compositions do not exhibit the orthorhombic-to-cubic transformation observed on CaMnO3 around 900oC, which, despite being completely reversible, might affect adversely the thermomechanical integrity of the structured perovskite bodies. In parallel, they demonstrate significantly high heat capacities, between 0.8-1.5 J/grK in the aforementioned temperature range. TGA/DSC tests with powders under tailor-designed multi-cyclic protocols exceeding 500 cycles, have shown that several such doped compositions are capable of combined sensible-thermochemical storage energy density ≥ 450 kWh/m3 in the abovementioned high-temperature range, exceeding by far that of state-of-the-art molten salts. Therefore, these compositions are in principle suitable for the manufacture of monolithic porous perovskite structures like honeycombs and reticulated porous ceramics (RPCs or “ceramic foams”) for further testing of the concept set forth.