BUT WHAT ABOUT THE DAYS WHEN THERE IS NO 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 0.33kW 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?"

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