Typical: A dam is built in a river. When the water flows through turbines down river, it generates electricity.
By pumping water back into the dam from down stream, energy can be stored. The typical solution has been in implementation since 1907.
Variations: The fluid does not have to be water, other fluids work better. The dams do not have to be in a river. All that is needed is two holding areas, with sufficient differences in elevation.
These different variations are in different stages of implementation.