Abstract
Thermochemical cycles are a type of heat engine that utilize high-temperature heat to produce chemical work. Like their mechanical work producing counterparts, their efficiency depends on the operating temperature and on the irreversibility of their internal processes. With this in mind, we have invented innovative design concepts for two-step solar-driven thermochemical heat engines based on iron oxide and iron oxide mixed with other metal oxide (ferrites) working materials. The design concepts utilize two sets of moving beds of ferrite reactant materials in close proximity and moving in opposite directions to overcome a major impediment to achieving high efficiency—thermal recuperation between solids in efficient countercurrent arrangements. They also provide an inherent separation of the product hydrogen and oxygen and are an excellent match with a high-concentration solar flux. However, they also impose unique requirements on the ferrite reactants and materials of construction as well as an understanding of the chemical and cycle thermodynamics. In this paper, the counter-rotating-ring receiver∕reactor∕recuperator solar thermochemical heat engine concept is introduced, and its basic operating principles are described. Preliminary thermal efficiency estimates are presented and discussed. Our results and development approach are also outlined.
Highlights
Solar and nuclear energies are the world’s only viable long-term energy options, and hydrogen production from these sources is potentially an environmentally advantageous long-term alternative to fossil fuels
The ferrite cycles utilize cyclic thermal reductionTRand water hydrolysis reactions with an iron-based metal oxide spinel to split water. They are attractive because they involve a minimum number of steps and reactants, have solid-gas reactions, use noncorrosive materials, lend themselves to direct solar irradiation of the working material, and can avoid the recombination reactions and irreversibility associated with quenching needed with volatile-metal oxides such as zinc or cadmium oxides
Recuperation of the sensible heat between the hydrolysis and reduction reactors is essential for high efficiency, especially for designs in which the ferrite is supported
Summary
Solar and nuclear energies are the world’s only viable long-term energy options, and hydrogen production from these sources is potentially an environmentally advantageous long-term alternative to fossil fuels. Thermochemical cycles are heat engines that utilize high-temperature heat to produce chemical work in the form of hydrogen5͔. The metal oxide cycles are attractive in that they involve fewer and less complex chemical steps than lower temperature processes, thereby resulting in less irreversibility and potentially higher cycle efficiency. The ferrite cycles utilize cyclic thermal reductionTRand water hydrolysis reactions with an iron-based metal oxide spinel to split water. They are attractive because they involve a minimum number of steps and reactants, have solid-gas reactions, use noncorrosive materials, lend themselves to direct solar irradiation of the working material, and can avoid the recombination reactions and irreversibility associated with quenching needed with volatile-metal oxides such as zinc or cadmium oxides. Kodama et al ͓14,15͔ and Ishihara et al ͓16͔ demonstrated that
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