There is a riddle at the heart of the renewable energy revolution. When the wind blows, the sun shines, and the waves roll, there is abundant green power to be generated. But when skies darken and conditions are calm, what do we do?

The answer, today, is to ramp up conventional power production, supplying the grid by burning fossil fuels. It is a 20th Century solution to a 21st Century problem – one that sits in sharp contrast with plans for carbon neutrality.

A cleaner future will mean focusing on ever-larger lithium-ion batteries, some energy experts say. Others argue that green hydrogen is the world’s best hope. And then there are those placing their bets not on chemistry, but the limitless force that surrounds us all: gravity.

“What goes up, must come down” – this is the immutable Newtonian logic underpinning gravity batteries. This new field of energy storage technology is remarkably simple in principle. When green energy is plentiful, use it to haul a colossal weight to a predetermined height. When renewables are limited, release the load, powering a generator with the downward gravitational pull.

A similar approach, “pumped hydro”, accounts for more than 90% of the globe’s current high capacity energy storage. Funnel water uphill using surplus power and then, when needed, channel it down through hydroelectric generators. It’s a tried-and-tested system. But there are significant issues around scalability. Hydro projects are big and expensive with prohibitive capital costs, and they have exacting geographical requirements – vertiginous terrain and an abundance of water. If the world is to reach net-zero, it needs an energy storage system that can be situated almost anywhere, and at scale.

Gravitricity, an Edinburgh-based green engineering start-up, is working to make this a reality. In April last year, the group successfully trialled its first gravity battery prototype: a 15m (49ft) steel tower suspending a 50 tonne iron weight. Inch-by-inch, electric motors hoisted the massive metal box skyward before gradually releasing it back to earth, powering a series of electric generators with the downward drag. 

The demonstrator installation was “small scale”, says Jill Macpherson, Gravitricity’s senior test and simulation engineer, but still produced 250kW of instantaneous power, enough to briefly sustain around 750 homes. Equally encouraging was what the team learned about their system’s potential longevity.

“We proved that we can control the system to extend the lifetime of certain mechanical components, like the lifting cable,” says Macpherson. “The system is also designed so that individual components can be easily replaced instead of replacing the entire system throughout its lifetime. So there’s real scope for having a decades-long operational life.”

While the Gravitricity prototype pointed upward, the company’s focus is now below ground. Engineers have spent the last year scoping out decommissioned coal mines in Britain, Eastern Europe, South Africa, and Chile. The rationale, explains managing director Charlie Blair, is pretty straightforward: “Why build towers when we can use the geology of the earth to hold up our weights?”

It seems like a neat solution. The globe is pockmarked with disused mine shafts deep enough to house a full-sized Gravitricity installation, which will stretch down at least 300m (984ft), and possibly much further. There’s political will to make it happen too, Blair says, with policymakers keen to tap into public enthusiasm for a so-called “just transition” – the notion of a new, low-carbon economy that secures the livelihoods of fossil fuel workers and their communities. And so, with enough funding, a subterranean prototype (most likely located in the Czech Republic) should be functioning by 2024. First, though, a series of challenges must be overcome.

“We need to look closely at the existing civil structures – the shaft lining, the shaft’s surroundings – and make sure they’re absolutely sound and capable of holding up several thousand tonnes,” Blair explains. “There are also potential safety issues around methane gas, and the mines being flooded.”

With that in mind, Gravitricity is also looking at sinking its own purpose-built shafts: an endeavour that’ll cost more upfront, but promises far greater uniformity further down the line.