The gently undulating hills of the British countryside are soon set to be green in more ways than one, after a deal was inked to use new hydropower technology in a bid to turn them into renewable energy-storing batteries.

The UK’s Mercia Power Response has signed a deal with London-based start-up RheEnergise to roll out its new High-Density (HD) Hydro energy storage system, which can harness excess power from wind and solar facilities.

The companies will first identify sites for the “new and innovative form of long-duration hydro-energy storage” that RheEnergise has pioneered, with an aim of rolling out 100MW in storage capacity by 2030.

At times of low energy demand, the HD Hydro system pumps a patented fluid two and a half times denser than water uphill to underground storage tanks larger than an Olympic-size swimming pool.

When energy demand and prices rise, the fluid is released downhill and passes through turbines, generating electricity to supply power to the grid. Projects will range from 5MW to 100MW of power and can work with vertical elevations as low as 100 metres or less.

“It means that HD Hydro can be deployed beneath the surface of hills rather than mountains,” say Mercia Power and RheEnergise, “opening up massive opportunities in the UK and around the world.”

RheEnergise says it has identified around 6,500 potential sites for its storage facilities across the UK, which, particularly in England, does not have many mountains to speak of. The company says mines and quarries could host its facilities as these have high elevations and are high energy users.

HD Hydro systems can be built in just a few years, says the company, compared to the average five to 10 year time it takes to build a traditional hydropower facility.

RheEnergise boats that its HD Hydro facilities will have a “dramatically reduced footprint” compared to traditional pumped-storage hydroelectricity facilities, which require the flooding of valleys.

RheEnergise commercial director Sophie Orme said that, with the “growing pressure to speed up the decarbonisation of the UK’s energy system,” it is significant that their facilities can gain planning consent in “months rather than years, so we are able to make a meaningful and positive impact on the energy transition over the next decade.”