Pumped hydro first emerged in the late 19th century. During subsequent decades, countries including the US and UK built lots of large plants, though construction had waned by the 1990s. The tech was originally designed to complement fossil fuel power plants, making use of excess energy they produced. But today grid operators increasingly value pumped hydro plants as workhorses able to mediate highly variable wind and solar assets. They can fill in shortfalls in electricity generation or soak up surplus energy within minutes, and store it for short or long periods.
Currently, significant amounts of energy go to waste because there’s no way to consume it at the moment of generation. The UK, for one, has squandered more than £1 billion ($1.32 billion) this year alone by turning off wind turbines due to lack of energy demand.
Pumped hydro plants could help to solve this problem but building them has often been expensive and difficult. RheEnergise’s denser-than-water fluid means the firm can pack more potential energy into a smaller space and at lower elevations. To replicate the firm’s 500 kilowatt (kW) demonstrator with a water-based version, for example, you’d need more than twice the volume of liquid they’re using, and the upper reservoir would need to be elevated to 200 meters rather than 80.
Vast reservoirs and towering mountains, historically associated with pumped hydro projects, may no longer be essential. “In somewhere like the UK, we think there are probably 20–25 viable sites for traditional pumped hydro, whereas for us it’s 6,500,” says Crosher. There could be hundreds of thousands of potential locations for the company’s technology worldwide—if RheEnergise can prove it works as intended. The firm tells WIRED it generated the first power from the system this week. Should the results from these tests continue to satisfy, a commercial-scale 10 MW project could follow by 2028.
RheEnergise is just one example of how pumped hydro is evolving. “It’s a really exciting time,” says Rebecca Ellis, senior energy policy manager at the International Hydropower Association (IHA), a nonprofit membership organization. The IHA estimates that 600 GW of pumped hydro projects are in the global pipeline. 8.4 GW was installed in 2024, she adds. One project that helped boost that total was the 3.6 GW pumped hydro plant in Fengning, China. It’s the biggest such facility, in terms of power capacity, in the world.