On the night of June 16, 2020, the United Kingdom achieved a remarkable achievement. While not a spectacular headline-grabbing feat, Guinness Book of Records-Worried guy, it was much more transcendental. The country had reached the end of a streak of 67 days, 22 hours and 55 minutes in which it had met all its energy needs without using coal, something it had not achieved for almost 140 years.
Although the first Covid lockdown undoubtedly played its part in drastically reducing the nation’s energy consumption during that period, this was an important milestone. With the race to net zero on people’s minds, it served as a timely reminder of the International Energy Agency’s forecast that renewable energy will become the largest source of electricity generation worldwide by 2025.
The target has The fundamental technical problem must be solved before the UK can achieve its clean energy revolution. The problem is that renewable energy supply can’t always match demand, so all the surplus megawatts generated by the country’s many solar and wind farms must be stored somewhere until they’re needed.
In the spring of 2021, a possible solution to this problem could be found at an industrial site in the port of Leith, Edinburgh. The 15m tall steel lattice tower looked very at ease in its surroundings, but he was not there to unload cargo from the ships in the harbor. She was there to store green electricity.
This so-called gravity battery was a scaled-down prototype created by Gravitricity, a Scottish startup that aimed to find out what a full-size version could accomplish.
“The purpose of this demonstrator was to test our technology in a real environment, verify response speed, and confirm our model,” explains Jill Macpherson, the company’s senior test and simulation engineer. “It allowed us to measure the performance of a real networked system, compare it to expectations, and learn technical lessons at little cost.”
The system worked by using excess electricity generated by solar panels to drive motors that lifted 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 fall (at a highly controlled rate), this turned the motors into generators that fed electricity back into the grid.
“The demonstrator was rated at 250kW, enough to power around 750 homes, albeit for a very short time,” says Macpherson. “But it confirmed that we can deliver full power in less than a second, which is valuable for operators who need to balance the network on a second-by-second basis. It can also deliver large quantities more slowly, so it’s very flexible.”
In this way, the gravity battery can store much of the solar energy that is generated during the day, when household demand is relatively low, and then release it at night, when consumption peaks.
According to an assessment carried out by researchers at Imperial College London before the pandemic, Gravitricity’s system offers energy storage at a cost of £137 per MWh averaged over 25 years. That’s less than half the cost of a comparable setup with lithium-ion batteries: £293 per MWh, an estimate that doesn’t take into account the ethical and environmental costs incurred in producing them.
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.
“In the demonstrator, we proved that we could control the system to extend the life of certain components,” says Macpherson. “For example, we test control methods to reduce peak forces and maximize the number of lifting cycles the cable can tolerate. The system is also designed so that components can be easily replaced, so there is a real chance it will have an operational lifespan of decades.”
Gravitricity plans to use abandoned mine shafts, hundreds of meters deep, that would be repurposed to house large-scale batteries.
The irony of the idea that old coal mines can help supply the nation with renewable energy is not lost on company CEO Charlie Blair.
“Large-scale energy stores in old mines can make good use of existing infrastructure and create jobs exactly in those areas where they are most needed,” he says. “The emotional aspect of this is also important. Entire communities once worked in the mines and are generally very happy to see them being used to store renewable energy.”
There is also the possibility that the batteries become lodged in modular buildings, each with thousands of pesos, with the design of each building corresponding to a specific energy demand of the network to which it is connected. A tall, skinny tower could provide a lot of power in a relatively short time, for example. But, if the footprint of that building were increased, the period during which that energy could be released would also be lengthened.
These structures could be built in many more places than, say, pumped hydro systems, which require much more land and are restricted to stretches of upland with abundant water supplies.
With enough forethought, there is even an opportunity to incorporate gravity batteries into the design of new tower blocks.
“There is also great potential to improve storage capacity by increasing the density of the material being lifted,” says Asmae Berrada, professor of energy at the International University of Rabat, Morocco. “It could even be made from recycled materials, which would significantly reduce the cost of the system. We are currently building a prototype using scrap steel, for example.”
The prospects for this rapidly developing technology look promising. At a time when we are desperate for certainty in our quest for a clean energy-powered future, what could be more reassuring than having a supply that is primed and ready to launch on demand? After all, what goes up must come down.