A grand challenge for using intermittent renewable energy such as solar for baseload applications is large-scale energy storage. Here, we propose an efficient means of implementing carbon recirculation cycles that enable dense energy storage. In these cycles, during the period of renewable energy availability, a suitable carbon molecule is synthesized from the stored liquid carbon dioxide and then stored in a liquid state. Subsequently, when renewable energy is unavailable, the carbon molecule is oxidized to deliver electricity and carbon dioxide is recovered and liquefied for storage. We introduce exergy based metrics to systematically identify candidate carbon molecules for the cycle. Such a search provides us the trade-off between the exergy stored per carbon atom, exergy used to synthesize the molecule and the exergy stored per unit volume. While no carbon molecule simultaneously has the most favorable values for all three metrics, favorable candidates identified include methane, methanol, propane, ethane and dimethyl ether. For cases where the molecule to be stored is gaseous under ambient conditions, we suggest synergistic integration between liquefaction and boilup of this gas and that of recirculating carbon dioxide. This unique feature allows for minimizing the energy penalty associated with the recovery, purification and liquefaction of carbon dioxide and storage of carbon molecules. Using process simulations we show that these cycles have a potential to provide GWh of electricity corresponding to an overall energy storage efficiency of 55-58% at much reduced storage volumes compared to other options. © 2014 Elsevier Ltd.
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