The use of intermittent or variable sources of energy, such as solar and wind energy, and some of the forms derived from moving water, often requires some means of energy storage.

Energy storage can not only potentially bene™t solar energy systems as well as other renewable energy resources but also bene™t the transmission and distribution systems because storage applications can be used to mitigate diurnal or other congestion patterns and, in effect, store energy until the transmission system is capable of delivering it where needed.