The Inside Story Of The World’s Biggest ‘Battery’  

Think Progress has an article on pumped hydro storage in the US - The Inside Story Of The World’s Biggest ‘Battery’ And The Future Of Renewable Energy.

The largest battery in the world has sat quietly in George Washington National Forest along the Virginia-West Virginia border for nearly 30 years. A five-hour drive from the nation’s capital, it sits in the middle of the Appalachians, tucked behind the Blue Ridge Mountains. ... The Bath County Hydro Pumped Storage Facility is not really a battery in the common sense of the term, but it is the largest pumped storage facility in the world. It stores a lot of energy, which helps 60 million people in 13 states (and DC) served by the regional transmission organization, PJM Interconnection. ...

Europe continues to invest heavily in pumped storage, with skyrocketing renewable output and decent capacity. There are 40 pumped storage facilities in the U.S., and Bath County is one of the youngest, even though it remains the largest in the world. Rocky Mountain Hydroelectric Plant in Georgia is the newest, commissioned in 1995. Development in the U.S. has been slow because the facilities are expensive, hard to site, and hard to build.