We propose a cycle to store and supply GWh of electricity from intermittent renewable energy by transforming carbon atoms between carbon dioxide and methane. Among hydrocarbons, methane is associated with the highest energy content per mole of carbon. Therefore, for a given electricity supply, it requires the least amount of carbon circulation. To minimize storage volumes, both carbon dioxide and methane are stored as liquids. When renewable energy is available, methane is synthesized from vaporizing carbon dioxide, and subsequently purified, liquefied and stored. When renewable energy is unavailable, liquid methane is vaporized and oxidized for electricity supply. The produced carbon dioxide is purified and liquefied for storage and subsequent usage. In each case, the electricity required for purification and liquefaction is minimized by integrating the vaporization and liquefaction steps of carbon dioxide and methane. The cycle can achieve 55% storage efficiency with much reduced storage volumes compared to other options. (C) 2014 Elsevier Ltd. All rights reserved.