Exploitation of thermochemical cycles based on solid oxide redox systems for thermochemical storage of solar heat. Part 4: Screening of oxides for use in cascaded thermochemical storage concepts

Kurzfassung

Air-operated Solar Tower Power Plants store the excess solar energy during on-sun operation as sensible heat in porous solid materials that function as recuperators during off-sun operation. Their storage capacity can be extended by coating or manufacturing the porous heat exchange modules with oxides of multivalent metals undergoing reduction/oxidation (redox) reactions accompanied by significant heat effects (e.g. Co3O4/CoO, BaO2/BaO, Mn2O3/Mn3O4, CuO/Cu2O). Furthermore, to maximize the amount of redox material that can be thermochemically exploited efficiently in a given thermochemical reactor volume, the idea of employing cascades of porous structures, incorporating different redox oxide materials and distributed in a certain rational pattern in space tailored to their thermochemical characteristics and the local temperature of the heat transfer medium is set forth. Thermogravimetric analysis studies with the oxides above were performed to identify the most suitable ones for further cascaded operation. The Co3O4/CoO redox pair has been already proven capable of stoichiometric, long-term, cyclic reduction–oxidation under a variety of conditions. The Mn3O4/Mn2O3 redox pair was found herein to be characterized by a large ‘‘temperature gap” between reduction (�940 �C) and oxidation (�780–690 �C) temperature, whereas the CuO/Cu2O and BaO2/BaO pairs could not work reproducibly and quantitatively. Thermal cycling tests with the Co3O4/CoO and Mn3O4/Mn2O3 powders operating together under the conditions required for complete oxidation of the less ‘‘robust” Mn3O4/Mn2O3, demonstrated that both powders can be reduced and oxidized in complementary temperature ranges, extending thus the temperature operation window of the whole storage cascade.