Pumped hydro has long been a workhorse of energy storage, reliable, proven, and capable of delivering grid-scale capacity. But traditional pumped hydro comes with a catch: it needs mountains and lots of water.
That’s the limitation RheEnergise wants to remove.
The UK-based company has developed what it calls High-Density Hydro, a pumped storage system that works not with water, but with a proprietary fluid that’s 2.5 times denser. That shift, says CEO Stephen Crosher, “turns the intermittency problem of renewables into a huge opportunity.”
RheEnergise is building its first full-scale demonstration project on a hillside near Plymouth, in the southwest of England. Construction began in June last year, and the company expects test results this summer.
Denser Than Water, Lighter on Landscape
The core idea behind RheEnergise’s technology is simple but powerful: replace water with a much denser fluid to store more energy in less space.
“Our fluid is a mineral suspension in water. It’s environmentally benign, non-toxic, very dense, but it’s a fully flowing liquid,” Crosher explained. “The density is 2.5 times that of water, so it’s just greater than the density of concrete. Concrete would float in our fluid.”
That higher density unlocks a key benefit: performance equivalent to traditional pumped hydro, but at much lower elevations. “Our performance is the same at 200 metres as you would get with water at 500 metres,” Crosher said. “There are many, many more hills in the world than there are mountains.”
The company estimates that with its fluid, projects can reduce elevation by 60% or cut the system’s physical footprint by 60%, or find a balance between the two. Either way, it means smaller, more flexible projects that can be located closer to energy demand.
“You can completely hide and mask a project if you’re in a sensitive landscape,” he added, pointing to a concept image of the landscaped site near Plymouth. “The projects can be pretty much entirely buried and hidden from view after they’re completed.”
The Engineering Edge
While the principle is based on conventional hydro, the details are anything but. RheEnergise’s fluid requires bespoke pumps, and turbines adapted for higher density and different flow characteristics.
“The architecture of the turbine is the same, it’s a Francis turbine architecture, but it does have to be re-engineered for the fluid in particular,” said Crosher. “There are elements of the Francis turbine design that don’t particularly work well with the mineral suspension.”
That process, of taking mature technologies and adapting them, has helped the company move quickly. “It’s an engineering activity, so not fundamental science,” he noted.
Their R&D centre in Montreal plays a key role in refining and validating designs. “We build, test, break. First of all, we would undertake some theoretical work… and then we’ll build a test rig and actually physically validate that theoretical work.”
One of the bespoke turbines used by RheEnergise.
A Market Looking for Storage
The global push toward renewables has exposed one of the grid’s central challenges: how to store energy from intermittent sources like wind and solar and deliver it when demand peaks. That’s where long-duration storage becomes essential.
RheEnergise is positioning its solution in the sweet spot between short-term batteries and ultra-long seasonal storage. “At something like an eight-hour duration, we’re about half the cost of lithium-ion batteries,” Crosher said.
He pointed to UK research suggesting that most energy shifting will happen in the 4–20 hour window, a segment where RheEnergise believes it can dominate. “Perhaps 60% of the total energy shifted by storage is in the part of the market where we have the greatest strength.”
Beyond that, the potential market is vast. “By 2040, the total global market in terms of CapEx is $4 trillion,” Crosher said, citing McKinsey research for the Long Duration Energy Storage Council.
Mines, Modular Builds, and Market Momentum
One of the company’s early focus areas is mining sites, both active and disused, which offer ready-made elevation, high energy demand, and in some cases, the raw materials to create RheEnergise’s proprietary fluid.
“Mines are very much a beachhead market for us,” Crosher said. “They have a high energy demand operating 24 hours a day, but also they fully understand what we’re doing… and we also have the potential of creating our high-density fluid from some of the waste materials.”
Interest is coming from around the world. The company has leads from more than 25 countries and signed memorandums of understanding with partners in Canada, the UK, Chile, and Australia.
One standout case is northern Chile, where solar generation has outpaced the grid’s ability to absorb it. “For around 1,600 hours per year, prices either go to zero or negative in the Chilean power market,” Crosher said. “That’s just because of the sheer amount of solar on the grid. That’s great for us, but it’s not good for the owners of those energy assets.”
The RheEnergise process can use smaller hills than traditional hydro.
Final Tests and Future Funding
The Plymouth project is now entering its commissioning phase. “We’re energising the site at the moment. We are commissioning the electrical sides of the generators, motors and drives next week,” Crosher said. “Results is very much a process that we’re expecting to do over the summer.”
As of now, the company has raised over £15 million, two-thirds in non-dilutive grants, with £2 million still available in its final seed round. “The share price is £15.76… we really want that to change, hopefully three, four times when we go to the institutional round at the end of the year.”
After that, the roadmap includes first-of-a-kind and follow-up projects in the UK, Ireland, Poland, and North America.
The vision remains simple: “High-Density Hydro. To locate projects on small hills rather than mountains. And to do that, we’re using a high-density mineral suspension.”
