Bulk energy storage can be incredibly useful in integrating variable renewable generation and providing ancillary services to the grid. The ultimate application of a particular energy storage technology is largely determined by its discharge time.
Traditionally, technologies with shorter discharge times tend to provide ramping and ancillary services (such as grid power quality regulation). Storage devices with longer discharge times often provide services that enable greater energy flexibility by shifting large loads or supplies over periods of several hours or more. These storage technologies have widely varying costs and commercial maturities.
Reproduced with permission from the Electricity Storage Association.
Electricity Storage Association. 2011. “Storage Technologies: Technology Comparison." link