UK engineers develop dense-fluid energy storage system suitable for low-elevation sites
When engineers at RheEnergise popped champagne in January, they were celebrating a milestone most people have never heard of: 500 kilowatts of peak power from a pumped-storage hydroelectricity system.
That technology has quietly underpinned the world’s power grids for over a century, accounting for more than 90 percent of global long-duration energy storage. But this version is different in one fundamental way.
It contains no water.
The fluid needed to be viscous enough to stay put during an accidental spill, yet flow as easily as water through pipes and turbines under normal conditions.
The world’s biggest battery — and its biggest limitation
Pumped-storage hydroelectricity runs on a beautifully simple principle. During off-peak hours, excess electricity pumps water uphill to a higher reservoir. When demand spikes, gravity pulls that water back down through turbines, generating power on cue. No combustion, no chemical degradation, no exotic materials.
That simplicity has made PSH extraordinarily durable. By 2023, global installed capacity reached nearly 200 gigawatts — more than 90 percent of the world’s long-duration energy storage. No other technology comes close at that scale.
The technology hits a hard geographic ceiling, though. “You need a suitable mountain, and you need to have a river running along the bottom,” says Tamas Bertenyi, cofounder and CTO of RheEnergise. “You also need an alpine valley you can dam up, and there are just not a lot of sites where you can do that.” Flat countries, rolling hills, densely populated regions — all effectively locked out of the world’s dominant storage technology.
That’s the problem RheEnergise set out to solve, according to IEEE Spectrum.
A fluid 2.5 times denser than water — and engineered to behave like it
The company’s answer is a proprietary substance it calls High-Density Fluid: a mineral-rich suspension 2.5 times denser than water. “It is so dense that if you threw a block of concrete into a pool of the fluid, it would float,” says Bertenyi. That density is the point — more energy potential per unit of height, which means lower hills can do work that mountains once monopolized.
Developing the fluid was harder than it sounds. RheEnergise worked with the University of Exeter, where cofounder Richard Cochrane was a professor of renewable energy systems, to engineer something satisfying two almost contradictory requirements. The fluid needed to be viscous enough to stay put during an accidental spill, yet flow as easily as water through pipes and turbines under normal conditions.
The solution was a shear-thinning, non-Newtonian formulation. At rest, the fluid is thick; under pumping pressure, its viscosity drops sharply — behaving, hydraulically, much like water. The mixture is 80 percent solid particulates by mass, suspended rather than dissolved. Spill it, and the particles simply dry and settle rather than seeping into soil or groundwater.
What the pilot project actually looks like
The pilot system, now running in the UK, keeps a deliberately low profile — literally. An upper reservoir sits at 80 meters of elevation. Two-and-a-half-meter fiberglass pipes carry the fluid down to a lower reservoir that Bertenyi describes as “basically a large swimming pool.” Both are buried underground, connected by steel pipe in a closed loop.
Above ground, almost nothing is visible. Only the powerhouse — housing the turbine, pump, fluid-management system, and electrical controls — breaks the surface. For communities sensitive to industrial infrastructure, that near-invisibility could matter considerably.
In January, the system hit its headline milestone: 500 kW of peak power. Modest, but it proves the concept works at real scale. The commercial ambition is far larger — RheEnergise envisions projects using two to four 5 MW modular turbines each, putting the target operating range at 10 to 20 MW per site. The company aims to deliver its first fully commercial system by the end of 2028.
Why gentler slopes could change where energy storage is built
The physics are straightforward. Higher fluid density means the same energy output is achievable from lower elevations and shallower gradients than traditional PSH demands, expanding the viable site map dramatically.
“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 Professor George Aggidis, emeritus professor of energy engineering at Lancaster University.
That matters for the broader grid challenge. Batteries are fast and modular, but cost-effective long-duration storage — the kind needed to balance renewable generation across eight to ten hours or more — remains elusive at scale. “Batteries alone can be expensive” for that role, Aggidis notes. RheEnergise’s target customers reflect the opportunity: independent power producers, utility companies, and energy-project developers all need exactly this kind of flexible, location-independent storage.
The road to commercialization — and the competition waiting
None of this arrives without friction. Aggidis is direct: “Larger scale deployment will require substantial civil works, permit requirements, and engineering coordination. This is more complex than plug-and-play battery systems.” Capital intensity and long lead times are familiar obstacles for any infrastructure-scale energy project.
The competitive field is also crowding fast. Sodium-ion and flow batteries are modular, quick to install, and falling in cost. Compressed-air storage, hydrogen, and thermal storage all compete for the same long-duration market. None has yet achieved the scale or cost profile of conventional PSH — but that landscape is shifting.
RheEnergise is moving on its own timeline, working with turbine manufacturers to develop modular units specifically engineered for the High-Density Fluid. The 2028 commercial target gives a concrete horizon to watch. If the company hits it, a technology that once required a mountain may soon need nothing more than a gentle hill — and that could quietly redraw the map of where clean energy storage is possible.
Carlos is an engineer with strong expertise in technical and industrial topics. He previously worked at international companies such as Siemens and is multilingual.
