Hydropower, used since ancient Greece and generating electricity for 120 years, is seeing renewed interest as a "water battery" to complement intermittent renewables. During surplus generation, electricity pumps water uphill, storing gravitational potential energy for later release. The largest North American facility, Bath County Pumped Storage Station, sits in Virginia's Allegheny Mountains.

London-based Rhe Energise is testing a high-density mineral-water mixture — "two and a half times the density of water" — enabling stations on hills rather than mountains. CEO Stephen Crosher says their fluid "runs freely like a creamy milk," offering speed and scalability with a smaller footprint.

Professor George Aggidis of Lancaster University notes pumped hydro provides large-scale, long-duration storage with long asset life and low emissions, but needs more flexibility for rapid renewables changes. Barriers include high upfront costs, geographic constraints, and environmental impacts.

International Hydropower Association CEO Eddie Rich emphasizes the need for 24/7 reliability as AI, EVs, and air conditioning strain grids. Global pumped hydro capacity surpassed 200 gigawatts last year, with a pipeline exceeding 621GW under development.