RheEnergise, an EIC Accelerator Winner, Pumps Heavy Fluid Uphill

Conventional pumped hydro needs around 500 metres of height and, in the words of RheEnergise's own report to the UK government, "decade-long timescales" to build. RheEnergise, a London company incorporated in 2019, keeps the principle and changes the liquid: its closed-loop system pumps a mineral suspension about 2.5 times as dense as water between two buried reservoirs, so that, by the company's account, a hill of 75 to 300 metres can do the same work.

The EIC Accelerator selected RheEnergise's project LDES-HDHydro at its March 2025 cut-off for a grant-only award, and CORDIS records an EU contribution of €2,499,999 for October 2025 to September 2027. The grant is not paying for the first plant. That was built near Plymouth with UK government money, and the government has published the company's final project report on it, which is considerably more detailed, and more candid, than the company's press releases.

A heavier fluid instead of a higher hill

The energy a pumped hydro plant stores is the weight of its liquid times the height it is lifted. Raising the density by a factor of 2.5 lets either the height or the reservoir volume shrink by the same factor. IEEE Spectrum describes the fluid as a proprietary mineral suspension of about 80% solid particles by mass, which thins when pumped and thickens at rest; the company's report to the UK government names barite powder as one of its ingredients. Both reservoirs are buried and the loop is sealed, which the report says needs 20% of the water volume of conventional pumped hydro and a smaller footprint.

The claims for commercial plants are the company's. The LoDES report, which the Department for Energy Security and Net Zero published and which RheEnergise wrote in the first person, says plants would be 60% smaller by volume than water-based ones, last more than 60 years and reach 80% round-trip efficiency, and that a typical project would be 40 MW with ten hours of storage. Its cost comparison, built on a model by consultancy Mott MacDonald, puts high-density hydro below lithium-ion batteries beyond about four hours of storage. None of these figures has been demonstrated at commercial scale, because no commercial plant exists.

The minimum height is described differently by different sources: tech.eu reported 100 metres or less in July 2025, the report gives 75 to 300 metres, and IEEE Spectrum puts the demonstrator's upper reservoir at 80 metres.

The Devon plant: what the LoDES report says

The UK government's Longer Duration Energy Storage (LoDES) programme awarded RheEnergise just over £8 million in September 2022 for a 500 kW demonstrator with four hours of storage, according to the report. The first site, at the Hemerdon mine, fell through in November 2023 over the mine's operating permits; a lease at Sibelco's china clay mine near Plymouth followed in April 2024, and surveys, design, planning and the grid application had to be repeated. The construction programme, compressed from 16 to 11 months to fit the funding rules, was extended in December 2023 to May 2025 and then to November 2025.

The report lists the problems on the way. Both reservoirs were cut in size by 50% to save money. The first batch of fluid did not match the design because one additive differed chemically from the tested version, which the report says its manufacturer knew about but had not flagged. A valve failure during the first operation of the fluid system required a clean-up of the powerhouse. The company wrote that its designer and owner's engineer, chosen for experience with small conventional hydro, "lacked the breadth needed" for the project, contributing to "late identification of defects".

The largest change came in October 2025. With the government programme closing, RheEnergise cut the fluid volume from 1,420 to 540 cubic metres, which the report says reduces running time at full power to about 15 minutes. It calls this a minimum viable product and "a compromise to the original scope". Production reached 32% of the design rate, slowed by damp barite powder, and 465 cubic metres were mixed to specification in just over five weeks. The report states an intention to produce the rest of the fluid later in 2026; whether that has happened has not been published.

The test results are positive within those limits. Over more than 100 pump and turbine operations, the turbine reached a peak efficiency of 89.6% with water and 84.8% with the dense fluid, and the measured efficiencies of each stage came within three percentage points of the computer models. The whole plant returned 59% of the energy put in, against a modelled 56%, before parasitic losses that the report says have not yet been measured. The company expects larger machines to close the gap to its 80% target; that is a projection, not a result.

The dates of full power differ slightly between sources. The report says commissioning with the dense fluid took place in December 2025 and ended in sustained operation at 500 kW. The company's press release of 27 January 2026 and ESS News reported full power that month, with the plant supplying the Sibelco mine at times of high demand.

Who paid, and how much is left

The UK figure is reported in different ways. ESS News gave £8.25 million; the report says the award rose from £8.2 million to £9.8 million, then fell by £64,500 when the fluid volume was cut, to £9,806,109. Total project spend reached £10.2 million, more than the funding, the report says, and it gives inflation, the change of site, the extended programme and unbudgeted extra work as the reasons.

The equity rounds that are public are small. Renewable Energy Magazine reported in February 2024 that the Low Carbon Innovation Fund put in £335,000 as the cornerstone of a £720,000 round, alongside a Crowdcube campaign. The company's news page announced a Canadian crowdfunding campaign on FrontFundr in May 2025 and a North American one on Republic in September 2025. A Form D filed with the US Securities and Exchange Commission on 7 April 2026 reports $158,090 sold to 63 investors in a $2,999,996 offering. The Republic total could not be read directly, and no amount for the FrontFundr campaign could be verified.

The Companies House filing history shows share allotments in January, March and December 2025 and in June and September 2026, and a charge created on 22 July 2026. The unaudited small-company accounts, which do not include an income statement, show net assets falling from £1,182,952 at the end of 2024 to £575,126 at the end of 2025, with £854,381 in the bank. They also record a convertible loan of £676,546 including interest, at 8%, secured by a debenture over the company's assets and repayable on 20 June 2026 unless converted; after the year end, the directors wrote, they began negotiating a six-month extension. How that loan was settled is not public. The directors also chose not to capitalise any development costs, because "the probability of future economic benefit is currently uncertain".

The report said in February 2026 that the company was pursuing more crowdfunding in the short term and "series A investment within the next six months". Searches of news coverage, the company's news page and the SEC's EDGAR database on 28 October 2026 found no announced Series A round.

What the EIC grant is for

According to CORDIS, LDES-HDHydro is to optimise and test the company's turbine for the dense fluid, finalise a manufacturing and scale-up strategy for a 5 MW prototype and take the technology to readiness level 8. The LoDES report describes the grant as funding the design of a 5 MW commercial-scale turbine, after which the company aims to build a 10 MW, six-hour first commercial project. Commercial plants are to use two or four 5 MW turbines, with the first fully commercial system targeted for the end of 2028, IEEE Spectrum reported; the January 2026 press release said within three years.

A year into the grant, CORDIS lists no deliverables or reports, which is normal at this stage. CORDIS and the LoDES report also disagree on patents: CORDIS cites 20 patents in 11 families, while the LoDES report counts 17 applications, of which 3 were granted.

1. Optimise and test the turbine for dense fluidRunningThe 500 kW demonstrator turbine reached 84.8% peak efficiency with the fluid, per the LoDES report; no output of the EIC project is published yet.
2. Design a 5 MW turbine and scale-up strategyNot yet reportedIEEE Spectrum reports work with turbine manufacturers; no 5 MW design or test result is published by October 2026.
3. Reach readiness level 8TRL 6 to 7The report claims TRL 7 for the demonstrator, which ran on a reduced fluid volume.
4. First commercial projectPipeline onlyCandidate sites in Poland, Scotland and Italy, per the report; none announced as financed.

Customers, sites and the European case

The report names mines and quarries, data centres, constrained grid locations and vertically integrated utilities as first customers, and says the company has signed three memoranda of understanding with developers on three continents and holds a pipeline it values at $1.75 billion of project capital across 35 opportunities. The candidate first projects it lists are a 10 MW, six-hour plant in south-east Poland at the environmental permit stage, two 10 MW plants on Scottish estates and sites in Italy. In October 2026 the company announced a memorandum with developer Ikigai Energy to assess sites starting in Italy, also reported by Energy-Storage.news. A memorandum is not a contract, and none of these projects has announced financing.

The project sits within EU storage policy. The European Commission's recommendation on energy storage of March 2023 asks member states to identify flexibility needs and ease permits for storage, and CORDIS tags the project 100% to climate action. The CORDIS summary sets the system against lithium-ion batteries, which it describes as covering only two to four hours and depending on constrained supply chains. The UK is an associated country in Horizon Europe, per Innovate UK Business Connect, and the UK government targets 4 to 6 GW of long-duration storage by 2030, per ESS News.

The reporting is based on publicly available sources, listed below.