Thermodynamic analysis of MgxFe3-xO4 redox CO2 conversion solar thermochemical cycle

The solar-to-fuel energy conversion efficiency (eta solar-to-fuel) of the MgxFe3-xO4 (x = 0.2-1.0) based CO2 splitting (CDS) cycle is estimated at steady reduction (Tred) and oxidation temperatures (Toxd) equal to 1673 K and 1273 K, respectively. The efficiency analysis is performed using the experimental results reported in the sol-gel-derived MgxFe3-xO4 based CDS cycle. The redox nonstoichiometry allied with the MgxFe3-xO4 during the reduction (delta red) and oxidation steps (delta oxd) is determined based on the experimentally obtained results. Efficiency analysis is conducted by considering the heat energy required to heat inert sweep gas and CO2. Heat penalty allied with the separation of the inert sweep gas from O-2 and CO2 from CO is also considered. The solid-to-solid heat recovery effectiveness (epsilon ss) is assumed to be zero, whereas the gas-to-gas heat recovery effectiveness (epsilon gg) kept steady at 0.5. The release of a high amount of O-2 and the production of an elevated CO level is responsible for the rise in the energy penalty associated with both separators. The obtained results also indicate that the total thermal energy required (Q?MgF-TC) to drive the cycle depends heavily on the sensible heat required (Q?MgF-sens) for raising the temperature of MgxFe3-xO4 from Toxd to Tred. The obtained results also show that eta solar-to-fuel depends heavily on the amount of CO produced and hence recorded to be the highest (4.3%) in the case of MgFe2O4.

Automated summary

The solar-to-fuel energy conversion efficiency of the MgxFe3-xO4 based CO2 splitting cycle was estimated at varying temperatures, showing that different redox steps can affect the efficiency. Various factors including the heat energy required to heat inert gas and CO2, as well as the heat penalty associated with gas separations, were taken into account in the efficiency analysis.