Storage has long been one of energy's biggest "Holy Grails." It holds the key to every significant move into smart grids.
The reason is pretty simple: If energy cannot be stored, it is lost. Even transferring it from one type of energy to another is of little consequence unless you can reverse the erosion in the entire system.
It's called entropy, and it's a pervasive problem.
Now, there are a number of rather complicated ways to explain this, but let's keep ours straightforward. Over time, a system ends up having more energy than can be readily used. This is always the biggest drain on any attempt to structure a more efficient approach.
So the basic question becomes how to "save" all of this energy from being lost.