Solar thermal energy storage via exploitation and rational combination of porous ceramic structures and redox oxides chemistry

Kurzfassung

The current state-of-the-art solar heat storage concept in air-operated Solar Tower Power Plants is to store the solar energy provided during on-sun operation as sensible heat in porous solid materials that operate as recuperators during off-sun operation. This storage concept can be rendered from “purely” sensible to “hybrid” sensible/thermochemical one, via coating the chemically inert porous heat exchange modules with oxides of multivalent metals for which their reduction/oxidation reactions are accompanied by significant heat effects (e.g. Co3O4/CoO, BaO2/BaO, Mn2O3/Mn3O4, CuO/Cu2O), or by manufacturing them entirely of such oxides. In this “hybrid” concept, solar-heated air produced during on-sun operation is used, in addition to sensibly heat a porous material, to power the endothermic reduction of the oxide with the higher metal valence state to that with the lower, like in the exemplary reaction schemes below; this thermal energy can be entirely recovered by the reverse, exothermic, oxidation reaction taking place during off-sun operation. 2 Co3O4 + ΔH  6 CoO + O2 …(1) ΔH=202 kJ/molreact 2 BaO2 + ΔH  2 BaO + O2 …(2) ΔH= 81 kJ/molreact 4 CuO + ΔH  2 Cu2O + O2 …(3) ΔH= 64 kJ/molreact 6 Mn2O3 + ΔH  4 Mn3O4 + O2 …(4) ΔH= 32 kJ/molreact The construction modularity of these current state-of-the-art sensible storage systems provides further for the implementation of spatial variation of redox oxide materials chemistry and solid materials porosity along the reactor/heat exchanger, to enhance the utilization of the heat transfer fluid and the storage of its enthalpy. Based on this characteristic, the concept of employing cascades of various 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 has been set forth and tested. At first, Thermo-Gravimetric Analysis on powders and small-scale, redox-oxides-coated honeycombs and foams have identified the most suitable ones for further testing, based on operational temperature range, advantages, limitations and peculiarities of redox operation. Subsequently, cascades of lab-scale, porous ceramic honeycombs and foams have been first tested with respect to their sensible-only storage capability in a specially built furnace test rig, then coated at various loadings with redox pair materials and comparatively tested with respect to sensible/thermochemical storage capability. Parametric studies involved quantitative comparison of such hybrid sensible/thermochemical storage systems vs. respective sensible-only ones via the measurements of air stream exit temperatures as a function of the oxygen uptake/release, air flow rate and kind of porous support and redox material employed. Finally, such sensible and thermochemical storage concepts were tested on a solar-irradiated receiver–heat storage module cascade. This concept of tailoring the porosity and chemistry characteristics of a thermochemical cascaded structure along a given thermochemical reactor volume can maximize the amount of redox material that can be efficiently exploited for thermochemical reactions, enhancing thereby the storage module’s volumetric heat storage capacity, transport, thermal and heat recovery properties and extending a solar plant’s off-sun operation beyond the current levels.