On the evening of 16 June 2020, the UK notched up a remarkable achievement. Although it wasn’t a spectacular feat of the headline-grabbing, Guinness Book of Records-troubling kind, it was much more momentous. The country had come to the end of a 67-day, 22-hour and 55-minute streak in which it had met all its energy needs without using any coal power – something it hadn’t managed for nearly 140 years.
Although the first Covid lockdown had undoubtedly played its part by drastically reducing the nation’s energy consumption over that period, this was still a significant milestone. With the race towards net zero very much on people’s minds, it served as a timely reminder of the International Energy Agency’s forecast that renewable energy will become the single largest source of electricity generation worldwide by 2025.
But a fundamental technical problem needs to be solved before the UK can achieve its clean-energy revolution. The problem is that the supply of renewable energy cannot always match the demand, so all the excess megawatts generated by the nation’s many solar parks and wind farms need to be stored somewhere until they are required.
In the spring of 2021, one potential solution to this problem could be found on an industrial site in the port of Leith, Edinburgh. The 15m-high latticed steel tower looked very much at home in its surroundings, but it wasn’t there to unload freight from ships in the harbour. It was there to store green electricity.
This so-called gravity battery was a scaled-down prototype created by Gravitricity, a Scottish startup that was aiming to work out what a full-sized version could achieve.
“The purpose of this demonstrator was to test our technology in a real-world environment, verify the speed of response and confirm our modelling,” explains Jill Macpherson, the firm’s senior test and simulation engineer. “It allowed us to measure the performance of a real grid-connected system, compare it with expectations and learn technical lessons at a reduced cost.”
The system worked by using excess electricity generated by solar arrays to power motors that hoisted a pair of 25-tonne weights on steel cables to the top of the tower. In effect, it converted the sun’s energy into gravitational potential energy. When the weights were allowed to drop (at a highly controlled rate), this converted the motors into generators that released electricity back into the grid.
“The demonstrator was rated at 250kW – enough to sustain about 750 homes, albeit for a very short time,” Macpherson says. “But it confirmed that we can deliver full power in less than a second, which is valuable to operators that need to balance the grid second by second. It can also deliver large amounts more slowly, so it’s very flexible.”
This way, the gravity battery can store much of the solar power that’s generated during the daytime, when household demand for it is relatively low, and then release it in the evening, which is when consumption peaks.
According to an assessment by researchers from Imperial College London before the pandemic, Gravitricity’s system offers energy storage at a cost of £137 per MWh averaged out over 25 years. That’s less than half the cost of a comparable set-up using lithium-ion batteries: £293 per MWh – an estimate that doesn’t account for the ethical and environmental costs incurred in their production.
The Leith prototype also offered several encouraging indications about the likely longevity of a full-scale version. Gravitricity estimates that its system could last 10 times longer than an equivalent lithium-ion battery.
“On the demonstrator, we proved that we could control the system to extend the lifetime of certain components,” Macpherson says. “For instance, we tested control methods to reduce peak forces and maximise the number of lifting cycles that the cabling can tolerate. The system is also designed so that components can be replaced easily, so there’s real scope for it to have an operational lifetime running into decades.”
Gravitricity is planning to use abandoned mine shafts, hundreds of metres deep, which would be repurposed to house full-scale batteries.
The irony in the notion that old coal pits can help to supply the nation with renewable energy isn’t lost on the firm’s MD, Charlie Blair.
“Full-scale energy stores in former mines can make good use of existing infrastructure and create jobs in exactly those areas where they’re most needed,” he says. “The emotional aspect of this is also important. Whole communities once worked in the mines – and generally they’re very happy to see them being used for storing renewable energy.”
There is also potential for batteries to be housed in modular buildings, each containing thousands of weights, with the design of each building corresponding to an energy demand specific to the grid to which it’s connected. A tall, slim tower could provide a lot of energy in a relatively short time, for instance. But, if the footprint of that building were increased, it would also lengthen the period over which that energy could be released.
These structures could be constructed in many more locations than, say, pumped hydro systems, which require far more land and are restricted to highland tracts with a plentiful supply of water.
With enough foresight, there is even an opportunity for gravity batteries to be incorporated into the design of new tower blocks.
“There is also huge potential for improving storage capacity by increasing the density of the material being lifted,” notes Asmae Berrada, professor of energy at the International University of Rabat, Morocco. “It could even be made of recycled materials, which would significantly reduce the system’s cost. We’re currently building a prototype using steel waste, for example.”
The prospects for this fast-developing technology look promising. At a time when we’re desperate for certainty in our quest for a future powered by clean energy, what could be more reassuring than having a supply that’s primed and ready for release on demand? After all, what goes up must come down.
