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This High-Density Hydro Storage System Ditches the Water
A new type of hydroelectric energy system that doesn’t use water was cause for the champagne to flow in January when engineers at RheEnergise in the United Kingdom succeeded in driving a pilot project to a peak power of 500 kilowatts. The system is a fresh take on pumped-storage hydroelectricity (PSH) power, a century-old technology first implemented in Switzerland in 1907 that has since been adopted globally and grown into a major form of energy storage. In 2023, pumped storage provided nearly 200 gigawatts in global installed capacity—over 90 percent of the world’s long-duration energy storage. Hence its nickname: the world’s biggest battery.
PSH works by pumping water up to a higher reservoir during periods of excess electricity from renewables or when demand from the grid is low, and letting the water flow back down under gravity through turbines to a lower reservoir when demand is high. The simplicity of the concept makes PSH efficient, cost-effective, long-lasting, and reliable with relatively low running costs once constructed.
“Pumped hydro is very mature,” says Tamas Bertenyi, a cofounder and chief technology officer of RheEnergise. “In terms of long-duration storage—let’s say 8 to 10 hours—it’s incredibly low cost. So there’s probably a hydro industry in most countries of the world.”
But PSH also has its downsides. Besides high upfront costs and long construction times, Bertenyi says the biggest disadvantage is its lack of scalability. “You need a suitable mountain, and you need to have a river running along the bottom. You also need an alpine valley you can dam up, and there are just not a lot of sites where you can do that.”
To make PSH scalable, RheEnergise has revamped the technology by constructing a closed-loop system and replacing water with a proprietary fluid it calls High-Density Fluid, which has 2.5 times the density of water. “It is so dense that if you threw a block of concrete into a pool of the fluid, it would float,” says Bertenyi.
In developing the fluid, RheEnergise worked with the University of Exeter in England, where Richard Cochrane (now deceased), a cofounder of the company, was a professor of renewable energy systems. The researchers sought to engineer a mineral-rich fluid that is not only much denser than water but has a manageable viscosity, is environmentally benign, and causes minimal abrasion or corrosion. That took “a lot of engineering and a lot of science,” says Bertenyi, because it raised two contradictory challenges: Have a low enough viscosity to flow like water but be dense enough to not go anywhere in the case of an accident.
How does RheEnergise’s High-Density Fluid work?
To reduce the fluid’s risk to the environment (from spills or entering the food chain), it’s formulated as a suspension mixture that suspends the particulate minerals, rather than dissolving them as a solution might. The fluid’s high density solved this problem: In the event of spillage, the particles will simply dry and settle, and not seep deep into soil or groundwater, according to Bertenyi.
RheEnergise formulated a dense yet low-viscosity fluid in its effort to make pumped-storage hydroelectricity possible in more places.RheEnergise
At the same time, the fluid—which is actually 80 percent solid particulates by mass—needed to have a viscosity as low as water to flow through pipes and turbines. Thus, the fluid was engineered to have a thick viscosity when it’s not moving, but have a decreased viscosity when pumped through a PSH system: a shear-thinning non-Newtonian behavior.
“Given the system can generate the same energy output from gentler slopes and lower elevations than traditional pumped hydro, it makes far more sites viable worldwide—including low hills and urban fringe areas—not just mountainous regions,” says George Aggidis, a professor emeritus of energy engineering at Lancaster University in the U.K. “And its long-duration storage makes it suitable for balancing generation by renewables, a gap where batteries alone can be expensive.”
The pilot project consists of a higher reservoir constructed at a height of 80 meters, with fiberglass pipes 2.5 meters in diameter feeding a shared chamber; while the lower reservoir is a simple concrete construction, “basically a large swimming pool,” says Bertenyi. Both reservoirs are buried underground and connected by a steel pipe to form a closed loop, leaving just the powerhouse containing the turbine, pump, fluid-management system, and the electrical control system visible.
“We expect our commercial projects to use two or four 5-megawatt turbines, so 10 to 20 MW is the sweet spot,” says Bertenyi. Having achieved peak power with its pilot project, he says the company is working with partners to bring the technology to commercialization, including turbine manufacturers that will produce modular turbines engineered to work with its fluid. The company aims to deliver its first fully commercial system by the end of 2028. Potential customers include independent power producers, utility companies, and energy-project developers.
But RheEnergise can expect to face some challenges along the way. Besides being capital intensive, “larger scale deployment will require substantial civil works, permit requirements, and engineering coordination,” says Aggidis. “This is more complex than plug-and-play battery systems.”
Then there’s the competition. Aggidis points to sodium-ion and flow batteries, which are modular, fast to install and rapidly decreasing in cost. Other emerging technologies include compressed-air energy storage, hydrogen storage, and thermal storage that are also seeking to get a foothold in the rapidly expanding energy-storage market.
This post was updated on 25 February 2026 to clarify that RheEnergise’s name for its proprietary fluid is High-Density Fluid. High-Density Hydro, which was originally used, is the name of the company’s overall system.
