The current work is a follow-up of the idea described in previous publications, namely of combining active thermochemical redox oxide pairs like Co3O4/CoO, Mn2O3/Mn3O4 or CuO/Cu2O with porous ceramic structures in order to effectively store solar heat in air-operated Solar Tower Power Plants. In this configuration the storage concept is rendered from “purely” sensible to a “hybrid” sensible/thermochemical one and the current heat storage recuperators to integrated thermochemical reactors/heat exchangers. In addition, the construction modularity of the current state-of-the-art sensible storage systems provides for the implementation of concepts like spatial variation of redox oxide materials chemistry and solid materials porosity along the reactor/heat exchanger, to enhance the utilization of the heat transfer fluid and the storage of its enthalpy. In this perspective the idea of employing cascades of various porous structures, incorporating different redox oxide materials and distributed in a certain rational pattern in space tailored to their thermochemical characteristics and to the local temperature of the heat transfer medium has been set forth and tested.
Thermogravimetric analysis (TGA) studies described in previous works have shown that the Co3O4/CoO redox pair with a reduction onset temperature ≈ 885–905°C is capable of stoichiometric, long-term, cyclic reduction-oxidation under a variety of heatup/cooldown rates. Further such studies with the other two powder systems above, described herein, have demonstrated that the Mn3O4/Mn2O3 redox pair is characterized by a large temperature gap between reduction (≈ 950°C) and oxidation (≈ 780–690°C) temperature, whereas the CuO/Cu2O pair cannot work reproducibly and quantitatively since its redox temperature range is narrow and very close to the melting point of Cu2O. Thus, a combination of two such systems, namely Co3O4/CoO and Mn2O3/Mn3O4 has been further explored. Thermal cycling tests with these two powders together under the conditions required for complete oxidation of the less “robust” one, namely Mn3O4/Mn2O3, demonstrated in principle the proof-of-concept of the cascaded configuration, i.e. that both powders can be reduced and oxidized in complementary temperature ranges, extending thus the temperature operation window of the whole storage cascade. A suitably designed test rig where similar experiments in the form of cascades of coated honeycombs and foams can be performed has been built and further such tests are under way.