Redox oxides-based solar thermochemistry and its materialization to reactor/heat exchanger concepts for efficient solar energy harvesting, transformation and storage

Abstract

Redox chemistries operating on the transition between the oxidized and the reduced state of an oxide of a metal exhibiting multiple oxidation states can be coupled with Concentrated Solar Power (CSP) facilities within the entire spectrum of solar energy harvesting chain, namely capture, transformation and storage. The common, high-temperature, reduction step of the oxidized oxide state can be performed in principle by solar-thermal-only means; however the required conditions are much milder if such an approach is aided via continuous removal of oxygen from the reaction environment via e.g. application of vacuum, solid state oxygen ions conducting materials, chemical reactions with reductive species, or a combination of these approaches. The oxidation step with water/carbon dioxide known as water/carbon dioxide splitting (WS/CDS), can be employed for the synthesis of fuels like hydrogen/syngas respectively, whereas oxidation with air, if accompanied by significant reaction enthalpies, can be used for thermochemical storage (TCS) of solar heat. Such processes involve not only gas-solid chemical reactions but heat exchange at high temperatures; therefore a major technical challenge lies in the proper design and operation of redox-oxide-based thermochemical reactors operating simultaneously and efficiently as heat exchangers. Such reactors/heat exchangers can employ the solid redox oxide as either moving “loose” particle/granule streams or as moving as well as non-moving porous structures, either directly exposed to solar irradiation or confined within containers decoupled from the solar receiver and being allothermally heated by the heat transfer fluid from it. In every such design, redox chemistry commonalities should be exploited with the synergistic use of the gas-solid heat exchange functionality. The presentation will address in this common perspective, material chemistry and reactor/heat exchanger design and manufacture issues critical towards the scale-up of these technologies. Results on single- and multi-metal mixed oxides based on abundant, inexpensive raw materials and shaped to granules, pellets, foams and honeycombs will be presented; such configurations were comparatively tested in resistively and IR heated lab-scale test rigs under very high temperature ramps to identify suitable concepts, materials and reactor configurations with respect to long-term cyclic performance and cost. These will be complemented by results from scaled up versions of respective structured and non-structured reactors, currently under solar field testing.