Energy storage is a necessary component of practical solar or wind energy systems. Without storage, a passing cloud or a day of calm winds could result in the lights turning off.

This article from philly.com discusses a proposed energy storage system that may be used to complement wind energy in Central Pennsylvania. The Bucks County-based Merchant Hydro Developers wants to convert 21 out-of-use anthracite coal mines into pumped storage facilities. When power is less expensive, intermittent wind power will be used to pump water into an upper reservoir. When energy prices rise during the middle of the day, the water will be released into the lower reservoirs of the mines, spinning turbines on the way down to generate a consistent and predictable flow of power.

Pumped hydro storage already accounts for the vast majority of stored energy in the world including 97% of the energy storage in the United States. The coal mine reservoir solution is unique because it is a closed system. Most pumped storage draws from flowing bodies of water  (e.g. rivers) and released the water back into the same system.