Exploitation of thermochemical cycles based on solid oxide redox systems for thermochemical storage of solar heat. Part 6: Testing of Mn-based combined oxides and porous structures

Abstract

Low-cost, environmental-friendly, oxide compositions capable of reversible reduction/oxidation under air with significant reaction enthalpies are the first prerequisite for eventual commercialization of thermochemical storage concepts in air-operated solar thermal power plants. Equally necessary however, is the shaping of such oxides into compact structures operating as integrated reactors/heat exchangers. In this perspective two Mn-based mixed oxide systems were investigated: a specific Mn2O3-Fe2O3 composition and selected Ca-Mn-based perovskite compositions CaMn1�xBxO3�d doped in the B site with Ti, Al or Mg. The particular (0.8)(Mn2O3) ⁄ (0.2)(Fe2O3) powder composition not only was reduced and re-oxidized in a fast and reproducible manner for 58 cycles under a wide range of heating/cooling rates in contrast to Mn2O3, but its re-oxidation was much more exothermic than that of Mn2O3. Furthermore the presence of Fe2O3 enhances the shapability of this system to foams; such foams also demonstrated cyclic redox operation maintaining their structural integrity for 33 cycles, not exploiting however all the amount of oxide used for their manufacture for the thermochemical reactions. The attribute of perovskites for continuous, quasi-linear oxygen uptake/release, can be beneficial to hybridization of thermochemical with sensible storage within a wider temperature range. Addition of Ti was found to have a beneficial effect on the perovskites’ redox stability. Whereas the shaping of such compositions to foams has not been attempted, CaMn0.9Ti0.1O3�d perovskite pellets exhibited Oxygen release/uptake per mass very close to that of the respective loose powder without structural degradation. However, the induced heat effects of the perovskites’ redox reactions are substantially lower and need to be improved in the perspective of commercial-scale applications.