Drop-in fuels from sunlight and air

Aviation and shipping currently contribute approximately 8% of total anthropogenic CO₂ emissions, with growth in tourism and global trade projected to increase this contribution further^1-3. Carbon-neutral transportation is feasible with electric motors powered by rechargeable batteries, but is challenging, if not impossible, for long-haul commercial travel, particularly air travel⁴. A promising solution are drop-in fuels (synthetic alternatives for petroleum-derived liquid hydrocarbon fuels such as kerosene, gasoline or diesel) made from H₂O and CO₂ by solar-driven processes^5-7. Among the many possible approaches, the thermochemical path using concentrated solar radiation as the source of high-temperature process heat offers potentially high production rates and efficiencies⁸, and can deliver truly carbon-neutral fuels if the required CO₂ is obtained directly from atmospheric air⁹. If H₂O is also extracted from air¹⁰, feedstock sourcing and fuel production can be colocated in desert regions with high solar irradiation and limited access to water resources. While individual steps of such a scheme have been implemented, here we demonstrate the operation of the entire thermochemical solar fuel production chain, from H₂O and CO₂ captured directly from ambient air to the synthesis of drop-in transportation fuels (for example, methanol and kerosene), with a modular 5 kW_thermal pilot-scale solar system operated under field conditions. We further identify the research and development efforts and discuss the economic viability and policies required to bring these solar fuels to market.