What is pumped hydro and why is it still the world's biggest battery?

Pumped-storage hydropower (PSH) uses electricity to pump water from a lower to an upper reservoir, then releases it through turbines when power is needed, at a round-trip efficiency of roughly 70%–85%. It remains the world's largest battery because global PSH capacity exceeds 170 GW — about 90% of all grid-scale storage — and single plants reach tens to hundreds of gigawatt-hours.

PSH exists to solve the temporal mismatch that defines any high-renewables grid. At midday on a clear day, solar output far exceeds demand and prices collapse, sometimes going negative; by early evening the sun is down, wind happens to be weak, demand is climbing and prices spike. PSH pumps water uphill on cheap power and releases it on expensive power, closing that gap in both commercial and physical terms. Reversible Francis turbines, which can run as a turbine to generate and as a pump to lift water, make them the natural machine for these plants.

Scale and lifetime are why nothing has displaced it. Against lithium-ion batteries, a PSH scheme reaches tens or even hundreds of gigawatt-hours, far beyond any single battery project today; it lasts 50 to 100 years while lithium cells need replacing every 10 to 15; it has no electrochemical material depletion and no thermal runaway risk, and its marginal operating cost is very low. The price is a 10-to-15-year build time and unforgiving siting: you need two bodies of water or hillsides with real elevation difference, and geology that can carry a dam and underground tunnels.

The largest single storage facility on the planet is the Bath County Pumped Storage Station in Virginia's Allegheny Mountains: 3,006 MW of capacity and about 24,000 MWh (24 GWh) of energy, in service since 1985 and now approaching 40 years of operation. Its upper reservoir holds roughly 11 million cubic metres and the lower about 10.5 million, with a head near 380 metres; at full output it can run for some 8 to 11 hours, and it remains one of the most important flexibility assets on the eastern US grid.

Japan has among the highest PSH penetration in the world, with roughly 27 GW in operation, about 10% of its total generating capacity. As an island system with no connection to a continental grid, Japan must balance itself internally; through the 1970s and 1980s it built PSH heavily to absorb surplus nuclear output overnight — nuclear plants cannot cycle easily — and release it at the daytime peak. China is now running the largest expansion in history: its 2021–2035 medium- and long-term PSH plan targets more than 120 GW by 2030 and 300 GW by 2035, equal to roughly 70% and 1.8 times today's entire global PSH fleet respectively.

Water is both the deposit and the withdrawal; it is energy and it is memory; it is the oldest storage technology we have and still an irreplaceable foundation of the grid.

This question is covered in depth in The Full Spectrum Every Energy Source Explained — A Singapore Perspective，第九章 9.4 + 第十三章 13.4