"In some situations it's possible to imagine closed loop systems that produce CH4 from excess power and use it as storage instead of batteries"
There are other large scale storage systems that can do much better. "Power Mountain" in Wales is a good example of gravity storage. Water is pumped up to a high reservoir when there is surplus power (from wind mostly) and let out through a power plant when needed. Batteries convert electrical energy to chemical energy and back. Phase changing materials can be very efficient at storing thermal energy derived from renewable sources. sunamp.co.uk