As the world accelerates towards net zero, one of the biggest challenges remains how to store renewable energy efficiently and cost-effectively. RheEnergise, a UK-based company, has developed a solution – High-Density Hydro, a next-generation, long-duration energy storage system that’s redefining the future of pumped hydro. We sat down with Stephen Crosher, CEO, RheEnergise, to find out more.
A smarter, smaller solution to a global problem
Today the dominant form energy storage is pumped hydro storage, which provides 95% of global capacity. Pumped hydro has been around for decades, but RheEnergise has turned it into a form of energy storage that can be used almost anywhere globally.
RheEnergise’s High-Density Hydro system builds on the same core principle as conventional pumped hydro, but with vastly improved flexibility and efficiency, storing energy by pumping the proprietary high-density fluid to a higher elevation when supply exceeds demand and releasing it through turbines when energy is needed.
“The innovation lies in the fluid,” explains CEO Stephen Crosher. “R-19 is 2.5 times denser than water, non-toxic, and non-corrosive. That means our systems can operate on low hills rather than high mountains, opening hundreds of viable sites across the UK and globally.”
This approach eliminates the need for remote mountainous regions and significant rainfall, enabling long-duration storage even in water-scarce locations like Southern Europe, the Middle East, and Australia.
At the company’s Cornwood demonstrator site near Plymouth, RheEnergise’s first project site is now fully operational. Having achieved consistent, successful operating cycles, the company has significantly de-risked its High-Density Hydro technology and is now ready to deploy the solution in commercial-scale Long Duration Energy Storage (LDES) projects in the UK and worldwide.
“The fluid is a stable suspension of benign minerals in water,” Crosher adds. “Its high stability and low viscosity allow it to operate efficiently in a compact system, making pumped hydro faster, smaller, and more cost-effective than ever.”
This approach reduces construction costs, accelerates deployment timelines, and allows energy storage to be installed in a much wider range of locations. HD Hydro is designed to deliver medium-duration storage, typically between four and twenty hours, making it ideal for day-to-day balancing of solar and wind generation.
“This real-world test site is a critical step towards full commercial deployment,” Crosher continues. “It’s already providing invaluable data on system performance as we prepare for our first large-scale projects within the next two to three years.”
The company’s modular systems are designed for 10–100MW projects – compact enough to connect to existing grid infrastructure or co-locate with renewable energy assets such as wind and solar farms.
Because the R-19 fluid increases density 2.5-fold, projects are smaller, faster to build, and more cost-effective – cutting civil engineering costs by up to 65%. HD Hydro can deliver flexibility to national grids, balance renewable intermittency, and enhance energy security.
“Our goal is simple,” says Crosher. “To make low-cost, low-carbon energy storage available wherever it’s needed, supporting grid resilience, decarbonising industries, and accelerating the global transition to net zero.”
Global momentum
The completion of the Cornwood project marks a significant milestone on RheEnergise’s journey. As the company transitions from R&D to commercial rollout, its High-Density Hydro system promises a cleaner, more adaptable way to store renewable power, making long-duration energy storage more accessible, scalable, and sustainable.
RheEnergise is scaling quickly. The company has agreements in place to deploy its technology across the UK, Europe, Australia, and South America, and last year obtained a €2.5 million grant from the European Innovation Council (EIC) to support its journey towards commercialisation. With a commercial pipeline already valued in the billions of dollars and active enquiries from every continent, RheEnergise is tracking hundreds of opportunities across more than 25 countries, while focusing on a number of attractive markets closer to home that offer the greatest opportunities for deployment.
RheEnergise is a portfolio company of Low Carbon Innovation Fund 2 (LCIF2), managed by Turquoise International, which has supported the company through its development phase. The investment fits with LCIF2’s focus on UK climate technology innovation, helping to build companies that can export their products and services to a global market.
“We’re redefining what’s possible for pumped hydro,” says Crosher. “By thinking smaller, we’re creating something with truly global impact.”
