"Pumped hydroelectric storage (PHS) is the most established technology for utility-scale electricity storage and has been commercially deployed since the 1890s. Because most low-carbon electricity resources cannot flexibly adjust their output to match fluctuating power demands, there is an increasing need for bulk electricity storage due to the calls to mitigate global warming. This entry introduces the PHS technology, the pros and cons, its history, and the prospect." (Quoted from paper)