Redox-oxide-based modular structures of porous ceramic foams and honeycombs for efficient sensible/thermochemical solar energy storage

Abstract

The current, sensible regenerative heat storage in porous solid materials in air-operated solar thermal plants can be hybridized with thermochemical storage within the same volume via coating the heat exchange modules with oxides of multivalent metals undergoing reduction/oxidation reactions accompanied by heat effects (e.g. Co3O4/CoO, Mn2O3/Mn3O4) or by manufacturing them entirely of such oxides. In this way solar-heated air from the receiver in addition to sensibly heating the porous solid can induce the endothermic chemical reduction of the oxide from its state with the higher metal valence to that of the lower; the thermal energy can be entirely recovered by the reverse exothermic oxidation reaction (in addition to sensible heat) during off-sun operation. The construction modularity of these systems provides for the design of the entire storage reactor/heat exchanger as a structure with rational 3-D spatial variation of redox oxide materials chemistry and solid material porosity, tailored to the local temperature and flow conditions, to enhance the utilization of the heat transfer fluid and the storage of its enthalpy. Comparative sensible-only and sensible-thermochemical storage studies on a variety of redox-oxide-coated porous ceramic honeycomb and foam cascades have been performed from laboratory test rigs up to the level of a solar-irradiated receiver–heat storage module combination. The effects of oxide composition and porous support on heat storage efficiency, process cyclability and system longevity are presented and discussed in conjunction to issues for further research.