Another good source of CO2 for the methane synthesis would be the cement industry. These facilities run around the clock and produce huge amounts of CO2.
Grid scale bulk energy storage is still to expensive even with low cost batteries like the Aquion or Liquid Metal Battery (which has by the way been renamed to AMBRI http://www.ambri.com/)
The most promising technology for really large scale energy storage is the storage module from Gravity Power (http://www.gravitypower.net/index.aspx) or for even lower cost down to 1 €/kWh is the Lageenergiespeicher by Prof. Heindl (http://lageenergiespeicher.de/en/hhs-storage.html)
But this technology take some years to develop and until then the discussed storage options might be a bridging technology.