Energy Storage via Carbon-Neutral Fuels Made From CO $_{2}$, Water, and Renewable Energy

Fossil fuels are renewable only over geological time scales. The oxidation, via combustion, of considerable amounts of carbonaceous fuels since the dawn of the industrial revolution has led to a rapid accumulation of CO
$_{2}$
in the atmosphere leading to an anthropogenic influence on the Earth's climate. We highlight here that a versatile energy carrier can be produced by recycling CO
$_{2}$
and combining it chemically with a substance of high chemical bond energy created from renewable energy. If CO
$_{2}$
is taken from the atmosphere, a closed-loop production process for carbon-neutral fuels is possible providing an energy-dense and easily distributed storage medium for renewable energy. The rationale for reduced carbon or carbon-neutral energy carriers made from recycled CO
$_{2}$
is described, focusing on, for transport applications, their manifestation as energy-dense carbonaceous liquid fuels. Techniques for the separation of CO
$_{2}$
directly from the atmosphere are reviewed, and the challenges and advantages relative to flue-gas capture are discussed. Pathways for the production of carbonaceous fuels from CO
$_{2}$
are discussed. An integrated system is proposed where renewable energy is stored in the form of synthetic methane in the gas grid for supply to the power generation and heat sectors while methanol and drop-in hydrocarbon fuels are supplied to the transport sector. The use of atmospheric CO
$_{2}$
and water as feed stocks for renewable energy carriers raises the important prospect of alleviating a dependency on imported fossil energy with the associated large financial transfers. Their application in the transport sector yields a high-value end product. The synthesis and storage of carbon-neutral liquid fuels offers the possibility of decarbonizing transport without the paradigm shifts required by either electrification of the vehicle fleet or conversion to a hydrogen economy. They can be supplied either as drop-in hydrocarbon fuels for existing reciprocating and turbine-powered combustion engines or, at lower energetic cost and using simpler chemical plant, in the form of low-carbon-number alcohols which can be burned at high efficiency levels in optimized internal combustion engines. The suitability of these fuels for conventional engines enables the continued provision of globally compatible, affordable vehicles.