Re-energizing CO2 to fuels with the sun: Issues of efficiency, scale, and economics

• We demonstrate that solar fuels could meet the majority of global liquid fuel demand.
• The scale has precedent but will require decades to build and install.
• Resources and economics point to solar-to-fuel efficiency as a key metric.
• A minimum 10% solar-to-fuel efficiency should be targeted in research and development.

The interconnected challenges posed by the strategic and economic importance of petroleum and the increasing concentration of atmospheric carbon dioxide are now widely recognized. A transition away from the fossil fuel dominated global economy toward a system with ever-lower carbon intensity but commensurate with energy demand is necessary. Addressing these challenges requires thinking differently, beginning with recognizing that from a technological point of view, liquid hydrocarbon fuels remain an ideal transportable energy storage medium, and that CO2 and H2O are simply the energy-depleted, oxidized form of hydrocarbons. Further, it is instructive to acknowledge that conventional fossil fuels are in fact “stored sunlight”. This perspective suggests searching for large-scale options that convert and store sunlight via reactions of carbon dioxide and water to more usable and transportable forms as nature did, but do so as efficiently, effectively, affordably, and sustainably as possible. Conversion of solar energy to reduce CO2 into hydrocarbon fuels is an attractive option, but it is not without significant resource, economic, and technical challenges. Herein we present a general examination of resource and economic considerations and conclude that solar-to-fuel efficiency is a key metric that drives not only the potential impact (scalability) of a technology for storing (contemporary) sunlight and sequestering carbon above ground as energy dense fuels, but also the economics. We show that given high enough efficiency (>10%) energy conversion routes, supplanting a large fraction of global petroleum-derived liquid fuels with synthetic solar-fuels is challenging but nonetheless possible; indeed it is quite plausible.