(53c) Thermochemical Conversion of H2O and CO2 into Solar Fuels Via Metal Oxide Based Redox Reactions

Authors: 
Gharbia, S., Qatar University
Yousefi, S., Gas Processing Center
Folady, J., Qatar University
Alfakih, M., Qatar University
Jilani, M., Qatar University
Hussein Ali, M., Qatar University
Angre, P., Mumbai University
AlMomani, F. A., Qatar University

Due to the continuous increase in the population of world and drastic depletion of the fossil fuel reservoirs, it is highly essential to invest towards renewable energy technologies such as solar energy (storage, conversion and utilization). A two-step solar thermochemical H2O and/or CO2 splitting process which utilizes metal oxide based redox reactions is one of the promising ways of producing solar fuels such as solar H2 or renewable precursors for fuels such as solar syngas (a mixture of H2 and CO). This process includes the endothermic reduction of metal oxides at elevated temperatures by releasing O2 and oxidation of the reduced metal oxide by H2O, by CO2, or by the mixture of the two producing H2, CO or syngas.

In this study, computational thermodynamic analysis of the two-step metal oxide based solar thermochemical H2O and/or CO2 splitting process was performed with the help of commercial thermodynamic softwares. The thermodynamic equilibrium compositions for the metal oxide based solar thermochemical H2O and/or CO2 splitting process were determined at different experimental conditions. Influence of thermal reduction and H2O and/or CO2 splitting temperatures, pressure, and inert gas flow rate on thermodynamic equilibrium compositions was investigated in detail. In addition, variations in the reaction enthalpy and Gibbs free energy and conversion of H2O and/or CO2 into H2, CO, or syngas during solar thermochemical cycles were also studied in detail. The second law thermodynamic analysis of metal oxide based solar thermochemical H2O and/or CO2 splitting redox system was also performed. Effects of thermal reduction and H2O and/or CO2 splitting temperatures and solar concentration ratio (C) on solar absorption efficiency of the solar reactor (ηabs), solar energy input to the solar reactor (Qsolar), rediation heat losses from the solar reactor (Qre-rad), net energy absorbed in the solar reactor (Qreactor-net), rates of entropy produced in the solar reactor (Irr,reactor) and cooling units (Irr,cooling) were calculated and plotted. The solar-to-fuel conversion efficiency (ηsolar-to-fuel) for the metal oxide based solar thermochemical H2O and/or CO2 splitting process was determined and effect of heat recuperation, thermal reduction and H2O and/or CO2 splitting temperatures and C on ηsolar-to-fuel was examined. Various metal oxide systems were thermodynamically investigated for the solar fuel production via thermochemical H2O and CO2 splitting reactions and the obtained results will be presented in detail.

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