Some 450 kilometres north of Helsinki, Finland, lies a decommissioned mine. Despite its remote location, it is being keenly watched because it looks set to play a role in revolutionising our energy systems – though not for the reasons you might suspect.
The Pyhäsalmi mine used to yield wealth from zinc and copper, but it is about to monetise the power of gravity. As the deepest metal ore mine in Europe, it is an ideal spot for what’s known as a gravity vault. UK-based company Gravitricity plans to dangle a heavy weight down a mine shaft and connect the mechanism to a generator. It will store power as potential energy by pulling up the weight, then generate it again by letting it plummet.
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If that sounds surprisingly simple, that is exactly the point. Governments are wrestling with the epic challenge of the intermittency of renewable power: how to keep the lights on when the wind doesn’t blow and the sun doesn’t shine. So far, they have largely focused on expensive technologies like hydrogen, nuclear power and lithium-ion batteries. But what if we could solve the problem of intermittency – and cut bills and emissions too – with far more rudimentary methods?
The gravity vault could be just the start. Other companies are developing energy storage methods that involve such technological marvels as salt, sand, water and hot bricks. These disarmingly straightforward technologies, it turns out, could provide the answer to one of the most pressing challenges of our time.
The amount of energy we generate from renewable sources is rising fast as we seek to cut carbon emissions in order to mitigate the worst effects of climate change. The US gets roughly 23 per cent of its electricity from renewables – that is, wind, solar and hydro – while for Australia the figure is around 32 per cent. The UK does even better, with about 47 per cent of electricity coming from renewable sources. But wind and solar power are notoriously intermittent. So, as we come to rely on these sources, we need a way of storing the power they generate in times of plenty to see us through the cloudy, windless days.
Lithium-ion batteries can do the job. However, even though their cost is tumbling, they are still expensive and, when used to store large amounts of power, they only work for a few hours before they begin to lose their charge. Facilities crammed with thousands of these batteries are popping up around the world, yet these mega-batteries have drawbacks, not least that they typically depend upon rare, expensive metals. Some of these, such as cobalt, are mined in awful conditions that raise serious human rights concerns.
How to store renewable energy
Governments all over the planet are also keen on hydrogen as a way to store energy. The most environmentally friendly form, known as green hydrogen, is made using excess renewable energy to split water. The hydrogen is then stored, and burned cleanly later to produce electricity. It is a neat idea, albeit one propelled on a tsunami of PR cash from oil and gas firms. Such companies want to continue making money by producing hydrogen from their own extracted fossil fuels and then supplying it through the existing gas pipe network. The massive advantage of hydrogen is that it can store energy for long periods – proposals suggest the gas could be stored in underground caverns – and it will be a crucial part of our energy future. But masses of infrastructure will be needed to pipe it around, which makes it painfully expensive.
All of which explains why there is interest in less complicated solutions. One of the simplest ways to store energy is known as pumped hydro, which involves two lakes, one at the top and one at the bottom of a hill. Use cheap surplus electricity to pump water to the top lake, then allow it to flow downwards and turn a turbine when power is needed. It is extremely effective and relatively cheap. But not everywhere has a lake handy, and places that do tend to be scenic and rich in wildlife – not areas we particularly want to disrupt.
puts a fresh twist on the basic idea of harnessing gravity to store energy. Instead of using lakes and water, it plans to employ mine shafts like the one in Finland. There are plenty of them around. For instance, the company is also engaged in discussions to build its tech at sites in Germany and the Czech Republic. In 2022, it built an above-ground demonstrator rig with a couple of 25-tonne weights suspended by steel cables in Edinburgh, UK.
To be fair, not every community has an abandoned shaft nearby. But that isn’t necessarily a dealbreaker for gravity-based energy storage. In 2020, at the International Institute for Applied Systems Analysis in Austria and his colleagues suggested a , in which sand or gravel would be carted uphill, then allowed to race back down when electricity is needed.
It is too early to say whether the gravity-based ideas will really take off. But gravity vaults are far from the only simple energy storage solution in the offing.
Batteries made simple
Another idea is to take the concept of a battery and strip out the problematic bits. There are several variants on this theme, including batteries that replace costly lithium with cheap sodium Massachusetts-based has developed a rechargeable battery that relies on something equally mundane – rust. It works like this: when charging, a current electrochemically converts iron oxide – rust, to you and me – into iron, and releases oxygen as a gas. When discharging, the battery sucks oxygen back in from the air and turns the iron back to rust, generating a flow of electricity.
Each of Form Energy’s batteries is about the size of a washing machine – so not suitable for an electric car. But that is fine for grid-scale energy storage, where installations can cover large swathes of land. Form Energy says its rust batteries are optimised to store electricity for 100 hours at a lower cost than conventional batteries. Backed with more than $360 million in funding, the company has begun building its first battery factory in West Virginia.
We can’t talk about making energy storage simpler, though, without talking about heat. According to the International Energy Agency, – to warm homes and offices and to power the industrial production of food, drink, chemicals, plastics and more. So instead of devising more gizmos to store and generate electricity, could we just store heat instead?
The promise of thermal energy storage
Thermal Energy Storage (TES) doesn’t often make headlines, but a raft of firms have already caught on to its potential. Take , based in California, which is developing products that store energy for several days in a huge stack of clay bricks sandwiched between two heating elements, like toast in a toaster. The idea is to hook up those heating elements to renewable energy sources and grill the bricks at 1500°C. The heat can then either be used directly in industry or to produce steam that drives a turbine and spits out electricity again. For Rondo Energy, it is a big step towards fighting climate change. “I get a deep sense of hope when I look at this brick,” said John O’Donnell, Rondo Energy’s CEO, in , gesturing at one of the grey, heat-storing blocks on stage behind him.
These giant brick toasters would need to be located next to the factories they would serve, with heat being conveyed by an insulated pipe. The simplicity of the system makes it cheap: Rondo Energy estimates the cost of storing power this way to be about half that of green hydrogen. The company has already garnered from Microsoft and the Saudi oil giant Aramco, among others. On top of that, the US government has to install its superhot brick batteries at two factories in Kentucky and Illinois owned by the drinks company Diageo. “This project will demonstrate an industrial heat and power model system that could be replicated in many other sectors,” says Diageo’s Marsha McIntosh-Hamilton.
Europe is also warming to heat storage, with one pioneer being the firm . Despite being based in chilly Norway, it takes inspiration from the Noor power station near Ouarzazate in Morocco. There, a vast array of mirrors all reflect sunlight to the top of a tower, where it heats molten salt to around 1000°C – salt is used because it has such a high heat capacity. The stored heat generates power for the town, even after the sun goes down. Kyoto employs the same molten salt trick, but uses Norway’s plentiful wind energy as the input.
The first working Kyoto system, known as a Heatcube – even though it looks like a tangle of pipes – has been installed at the Nordjylland Power Station near the Danish city of Aalborg. It can store heat for days at a time and release it as electricity to meet variable demands on the grid. In 2023, the system designed to check that it could supply power at the required speed and consistency for the Danish electricity grid. Like Rondo Energy’s bricks, the Heatcube can also double as a way to provide industrial heat. “With all the excitement about battery technology for electric vehicles, people have forgotten about the massive demand for heat for industries,” says Kyoto’s chief technology officer, Bjarke Buchbjerg. “Industrial heat is a big deal – we can’t afford to ignore it.”
So where do we go from here? It is important to recognise that no single energy storage technology is a panacea. We will need plenty of different options to meet our needs, depending on local geography and many other factors, and it remains to be seen which solutions come to fore. says Simone Abram, director of the Durham Energy Institute in the UK.
That said, TES is having a moment. “Molten salt is going to be an important part of the energy mix,” says at the University of Sheffield, UK. “It’s a fantastic technology, offering high temperatures at industrial scale.”
Heat storage technology remains in its infancy and its affordability and efficiency is a slippery, contested matter. Yet we are also starting to get preliminary numbers on its costs. In , the International Renewable Energy Agency (IRENA) calculated the cost of TES using molten salt at around $22 to $26 per kilowatt-hour, whereas using hot bricks was $10 to $15 per kWh, depending on the scale and design. These costs are much lower than green hydrogen storage, which the US Department of Energy estimates at up to $50 per kilowatt-hour, largely because the infrastructure needed is so involved.
IRENA forecasts the global market for TES could triple in size by 2030, with investments reaching as much as $28 billion by the same date. In a February 2024 report, the environmental consultancy Systemiq said that deploying TES technology widely worldwide could cut energy-related greenhouse gas emissions by 14 per cent by 2050.
Despite piecemeal investment in TES, governments have been slow to get the message. Before the UK’s recent general election, the Conservative government had announced plans to plough £21 million into several low-carbon hydrogen projects. But there has been little action on TES, which is cheaper. Moreover, the UK House of Lords science and technology committee published a report in March tellingly titled “Long-duration energy storage: Get on with it”. It warned that the UK government lacks a robust plan to put in place long-term energy storage. “It is distressing to see that the [Conservative] Government lacks a clear plan for energy supply risks and indeed is still deliberating over investment in energy storage to prevent future crises,” said Julia King, chair of the committee, at the time.
Fledgling TES companies are also finding it hard to raise their voice above the better funded nuclear and hydrogen industries. At the Innovation Zero conference in London in April, a UK minister said that, at times of low wind and sunshine, electricity would in future be provided by new nuclear power stations. The statement baffled energy experts, who pointed out that nuclear is the last technology that you would want to use for backup: it isn’t the sort of thing you can turn off and on with the flip of a switch.
The sizeable backing provided by the European Commission, among others, for Rondo Energy’s projects in Denmark and Germany suggests the picture is beginning to change elsewhere. But energy experts insist governments need to go further, with specific strategies for storing heat for industrial processes and grid-scale electricity storage. “Electrochemical batteries have their place,” says at the University of Strathclyde, UK. “But the current fad for massive installations of them does absolutely nothing – and never will – for the desperate need for medium-duration and long-duration energy storage.”
Meanwhile, the simple energy storage concepts keep coming. Take Peter Dearman, a self-described “tinkerer” who became interested in sustainability at age 12. At the time, he was focused on using compressed air as an alternative to fossil fuels. It was a simple idea: use electricity to compress air into a super-cold liquid at -196°C (-320°F), store it in a tank and then warm and release the air to drive pistons.
Dearman has now been working on liquid air power for 50 years. In 2020, the firm he founded set up using liquid air to store power. It proved successful, so the next step is a larger facility that can store enough power to run 50,000 homes for 5 hours. Dearman says that if waste heat from a factory, say, is used to mobilise the super-chilled air, the system approaches the efficiency of batteries. “The beauty of liquid air storage is that it uses off-the-shelf simple technology made in the UK,” says Dearman. “I’m confident it will play a role in our energy future.”
Roger Harrabin is a freelance journalist and an honorary fellow at St Catharine’s College, University of Cambridge
