Assessing the Viability of GeO2/GeO Redox Thermochemical Cycle for Converting CO2 into Solar Fuels

Abstract

The solar thermochemical process of splitting CO2, known as CDS, is studied here using a redox cycle involving GeO2/GeO. The required thermodynamic data for a second-law-efficiency analysis is obtained from the HSC Chemistry software. The goal of this study is to investigate how different parameters, such as the operating temperatures and molar flow rate of the inert sweep gas, as well as the inclusion of separation units, heat exchangers, heaters, and coolers, can affect the solar-to-fuel energy conversion efficiency of the GeO2/GeO cycle. All calculations assume a constant gas-to-gas heat recovery effectiveness of 0.5. The analysis shows that the solar-to-fuel energy conversion efficiency is lower at a thermal reduction temperature of 1600 K (11.9%) compared to 2000 K. This is because high energy duties are required for heater-2, heater-3, and separator-1 due to the need for a higher inert gas flow rate. After conducting a comparative analysis of the three CDS cycles, it can be inferred that the GeO2/GeO cycle exhibits a significantly higher solar-to-fuel energy conversion efficiency in comparison to the ZnO/Zn and SnO2/SnO cycles across all thermal reduction temperatures. According to the comparison, it is confirmed that the GeO2/GeO CDS cycle can achieve a reasonably high solar-to-fuel energy conversion efficiency of 10% at less than 1600 K. On the other hand, ZnO/Zn and SnO2/SnO CDS cycles require a thermal reduction temperature of more than 1850 K to achieve a solar-to-fuel energy conversion efficiency of 10%.