I recently reviewed an EPRI document that discussed storage, and by far the largest size storage systems were pumped storage plants. I wondered why they did not include hydro (non-pumped) storage, as this form of storage is far larger than any other form of storage that is available on the grid now.
Parts of North America, but sadly not all of it, are blessed with mountainous territory that has many rivers and streams that run downhill, and many of these have been harnessed for electricity production. While not specifically intended as storage plants when built, the value of their storage may well turn out to be larger than the value of the electricity that they may produce.
Consider a hydro dam that is 35 M in height with a reservoir that is 10 km2. Discharging the top 1 M of water through a generating station (90% efficient) would release almost 840 MWh of stored energy. This is a small hydro plant, with a small reservoir behind it, yet the storage is almost 840 MWh/M of depth that is drawn from the forebay. That is in addition to the electrical energy generated for use.
So how does a utility that has no pumps manage to store and return energy? The process is both simple and efficient.