Sun-to-Fuel Assessment of Routes for Fixing CO2 as Liquid Fuel

This study presents a systems approach for comparing alternative routes for converting CO2 to liquid fuel using solar energy based on a novel metric of sun-to-fuel (STF) efficiency. The metric refers to the fraction of incident solar energy that is recovered in the liquid fuel. The STF efficiency analysis identifies energy and land use efficient routes that require immediate research and development effort to speed up their progress toward long-term cost-effectiveness. The analysis' unique insights are particularly relevant for densely populated regions, having scarce per capita land availability relative to the per capita energy demands. With atmospheric CO2 as the renewable carbon source, we present a detailed case study comparing the currently known photosynthetic routes with a theoretical route based on direct extraction of CO2 from air and its subsequent thermochemical conversion to liquid fuel. The findings indicate that the latter route could be potentially more energy and thereby land use efficient than any of the currently known photosynthetic routes, therefore, warranting its inclusion in any transportation fuels research portfolio. An interesting finding of our study is that for the interim period while CO2 extraction is still uneconomical and CO2 sourced from power plants is instead used, the relative energy efficiency of different routes remains unchanged. This suggests that the results in general are independent of the concentration of the CO2 source.