Exploitation of thermochemical cycles based on solid oxide redox systems for thermochemical storage of solar heat. Part 3: Cobalt oxide monolithic porous structures as integrated thermochemical reactors/heat exchangers

Abstract

In the perspective of thermochemical storage of solar energy via redox reactions of multivalent metal oxides, the manufacture and relevant testing of porous ceramic foams made entirely of Co3O4 was attempted, in order to maximize the amount of redox powder that can be incorporated in a given thermochemical reactor volume. Small-scale Co3O4 foams with satisfactory structural integrity were successfully produced. The foams were tested in cyclic reduction/oxidation conditions in a TGA apparatus in comparison to pellets made also entirely of Co3O4. Both these monolithic, porous, structures were capable of cyclic redox operation, exploiting for the thermochemical reactions the entire amount of redox material used for their manufacture. Full extent of reduction/oxidation was observed, in a fully reversible pattern. The initial density of the samples had an effect on specimen’s integrity: the much denser pellets could not retain their structural integrity, exhibiting cracks even after only two cycles. On the contrary, foams were tested for up to 15 redox cycles, maintaining simultaneously their structural integrity and stoichiometric redox performance. Dilatometry experiments under the same temperature-programmed conditions with the TGA ones revealed that during redox cycling, “chemically”-induced stresses are developed due to the expansion/contraction of the cobalt oxide lattice during oxygen release/uptake respectively. These stresses are superimposed to “thermal-only” ones due to temperature cycling and under certain circumstances can lead to structure deformation and fracture. In this respect “open” porous structures like the particular foams proposed and tested in this work have an advantage since their large void space can reversibly accommodate and “buffer” the large volume expansion much better.