Structured CaMnO -based porous ceramics for high-temperature, hybrid sensible-thermochemical storage in future Concentrating Solar Power plants

Abstract

Addressing the materials needs and requirements of next generation concentrated solar thermal (CST) technologies targeted on high-temperature processes for electricity generation and solar thermochemical applications, the present study reports recent research results on the shaping and relevant testing of porous monolithic structures made entirely out of CaMnO3-based perovskite powder compositions. The approach targets on the eventual use of such structures as hybrid sensible-thermochemical heat storage media, based on their characteristic capability for cyclic reduction-oxidation (redox) in direct contact with air, accompanied by significant endothermal/exothermal heat effects, complete reversibility of oxygen uptake/release and dimensional changes due to expansion/contraction. Following up prior work on synthesis of Ca-Mn-based perovskite powder compositions optimized with respect to the specific application, monolithic ceramic structures namely cylindrical honeycombs and reticulated porous ceramics (RPCs also known as ceramic foams) were produced. Honeycombs were produced via the industrially established extrusion technique that involves preparation of homogeneous “viscous pastes” of the ceramic powders that are then fed through a honeycomb die, creating parallel channels. Foams, on the other hand were prepared via the “replica” route, which involves impregnation of “sacrificial” polyurethane (PU) foam templates with slurries of the perovskite powder. Both approaches include subsequent drying and sintering of the shaped green body in order to “burn out” the binders/polymeric template respectively, and induce sufficient mechanical strength to the final ceramic piece. Several process parameters were optimized; e.g. the particle size of the perovskite powders employed in the preparation of pastes/slurries and the sintering conditions – ramp rate, sintering temperature and dwell time - with feedback from properties characterization results (e.g. porosity, mechanical strength, pressure drop). Lab-scale cylindrical honeycombs and foams (OD25 mm) of different channel/cell densities, were prepared entirely from such calcium manganite-(CaMnO3)- based powders. Both kinds of specimens were mechanically rigid to be further handled. Mechanical properties as a function of composition were determined from 4-point-bending tests on bar specimens and compression tests on honeycombs and foams. Both kinds of structures were experimentally validated for their thermochemical heat storage functionality in in-house tailor-designed and built test rigs upon cyclic electrical heating up to 1100oC under air flow, followed by cooling in nitrogen and isothermal oxidation in air at several temperature plateaus between 650-850oC. These tests demonstrated the ability of such structures to store and release heat defined by the enthalpy of a reversible redox reaction wherein the heat stored during endothermic reduction was reversibly released during exothermic oxidation. Furthermore, such heat effects during exothermic oxidation could be clearly manifested as a measurable temperature rise of both the honeycomb/foam specimen tested as well as of the air stream flowing through it, demonstrating, for the first time to the best of the authors’ knowledge, the ability of a perovskite to generate repeatable measurable heat effects. Current work involves the shaping of larger-scale monolithic structures for incorporation and operation in a “proof-of-concept-scale” modular unit to demonstrate the concept’s scalability.