Hybrid sensible/thermochemical solar energy storage concepts based on porous ceramic structures and redox pair oxides chemistry

Kurzfassung

The enthalpy effects of reversible chemical reactions can be exploited for the so-called thermochemical storage of solar energy. Oxides of multivalent metals in particular, capable of being reduced and oxidized under air atmosphere with significant heat effects are perfect candidates for air-operated Concentrated Solar Power plants since in this case air can be used as both the heat transfer fluid and the reactant (O2) and therefore can come to direct contact with the storage material (oxide). Based on the characteristics of the oxide redox pair Co3O4/CoO as a thermochemical heat storage medium and the advantages of porous ceramic structures like honeycombs and foams in heat exchange applications, the idea of employing such structures either coated with or entirely made of a redox material like Co3O4, as a hybrid sensible-thermochemical solar energy storage system in air-operated Concentrated Solar Power plants has been set forth and tested. At first, small-scale, redox-inert, cordierite foams and honeycombs were coated with Co3O4 and tested for cyclic reduction-oxidation operation in Thermo-Gravimetric Analysis studies. Such Co3O4-coated supports exhibited repeatable, cyclic reduction-oxidation operation within the temperature range 800-1000oC for many cycles, employing all the redox material incorporated, even at very high redox oxide loading levels. To further improve the volumetric heat storage capacity of such reactors, monolithic porous ceramic foams made entirely of Co3O4 were manufactured. Such foams sintered at 1300oC exhibited satisfactory structural integrity and were comparatively tested vs. the “plain” Co3O4 powder and the Co3O4-coated, porous cordierite supports under the same cyclic redox conditions up to 15 consecutive cycles. The Co3O4-made porous foams were proved also capable of cyclic reduction–oxidation, exploiting the entire amount of Co3O4 used in their manufacture, maintaining simultaneously their structural integrity.