Metal-oxide based thermochemical cycles, such as those including a class of iron containing materials commonly known as ferrites, involve two reaction steps: a thermal reduction at temperatures up to 1600 °C driven by a solar energy input, and a lower temperature exothermic oxidation in the presence of either carbon dioxide or water. In order to maximize performance, the reactive materials must be arranged into structures that provide an effective interface for the direct absorption of concentrated solar energy and also have relatively high surface area to support rapid chemical reactions. In this paper we discuss the attributes of reactive structures for solar thermochemical processes as well as some of the fabrication techniques currently under development at Sandia National Labs. One of these structures has been demonstrated on-sun in a two step carbon dioxide splitting cycle. The results, given in this paper, indicate that performance may be improved as the fraction of the total directly illuminated surface area is increased, reducing the need to rely on conduction or convection to distribute heat throughout the material.
Entropy Analysis of Solar Two-Step Thermochemical Cycles for Water and Carbon Dioxide Splitting
