In late November, engineers in Devon, England quietly began testing a novel twist on a century-old energy storage idea. At a former china clay mine near Plymouth, workers carefully mixed a mineral-based powder into water to create a fluid 2.5 times denser than water but still able to flow easily. This unusual liquid is the core of a demonstration project by British startup RheEnergise, which aims to rethink pumped hydro by dramatically reducing the space and elevation traditionally required.
The principle behind pumped hydro is simple and long established. Energy is stored by using excess electricity to move a fluid uphill into an upper reservoir. When electricity is needed, the fluid flows back down through turbines, generating power. First developed in the late 19th century, the technology expanded rapidly through much of the 20th century to complement fossil fuel plants. Although construction slowed by the 1990s, it is now experiencing a revival as grids struggle to balance highly variable wind and solar generation.
Modern electricity systems waste enormous amounts of renewable energy because supply and demand rarely match perfectly. In the UK alone, more than £1 billion worth of wind power has been curtailed this year. Pumped hydro plants can absorb surplus electricity within minutes and release it later, acting as flexible grid stabilizers over both short and long timescales. However, conventional projects often require large reservoirs and steep elevation differences, making them expensive and geographically limited.
RheEnergise’s dense-fluid approach could expand the number of viable sites dramatically. Because denser liquid packs more potential energy into less volume, the company’s 500 kW demonstrator operates with an 80-meter height difference instead of the 200 meters a water-based system would require. The firm claims this could increase potential sites in the UK from a few dozen to several thousand, enabling faster deployment of mid-scale storage. Having generated its first power, RheEnergise hopes to scale up to a 10 MW commercial project by 2028.
This innovation comes amid a global surge in pumped hydro investment. According to the International Hydropower Association, around 600 GW of projects are currently in the global pipeline, with 8.4 GW added in 2024 alone. Landmark facilities such as China’s 3.6 GW Fengning plant and Germany’s Goldisthal station demonstrate the technology’s immense scale and responsiveness. Goldisthal can ramp from zero to full output in 90 seconds and switch rapidly between generating and pumping, making it highly profitable in volatile electricity markets.
Yet large projects are not without challenges. Australia’s Snowy 2.0 expansion, set to deliver an extraordinary 350 GWh of storage, has faced major delays, environmental concerns, and costs ballooning to as much as AUD$18 billion. Such experiences highlight the risks of massive infrastructure projects, even as advocates argue their long lifespans justify the investment.
Experts broadly agree that pumped hydro remains the most deployable and proven form of long-duration energy storage worldwide. While traditional megaprojects will remain essential, emerging high-density and smaller-scale designs promise faster, more flexible deployment. Together, they suggest that old physics, paired with new engineering, could play a central role in accelerating the clean energy transition.