Salt, air and bricks: could this be the future of energy storage?

07 04 2024 | 17:14Roger Harrabin / THE GUARDIAN

Start-ups turn to heat over batteries as they aim to industrialise the practice

Think of battery ingredients and lithium, cadmium and nickel come to mind. Now think again. What about salt, air, bricks, and hand-warmer gel? In our electricity-hungry future they’re set to provide heat to manufacturers who need it, and to help keep the lights on at times when energy is short.

Energy storage has a dual purpose: it plugs gaps when the wind drops or the sun stops shining, and it allows users to buy cheap off-peak power and use it when they need it.

Until now, the focus of storage for industry has been mainly on giant conventional batteries, which the UK’s National Grid hopes to hook up to the grid more quickly amid delays. But there’s growing interest in storing energy in the form of heat – and that’s where the everyday ingredients such as air, salt and bricks come into the picture, because these materials are really good at holding warmth. A clutch of start-ups are now aiming to industrialise the practice.

Heat storage is coming up the agenda: last month a Lords committee urged the government to take energy storage more seriously, and this month a conference run by the Institution of Mechanical Engineers (IMechE) will hear the case for thermal batteries.

Heat provides more than half the total energy demanded by industry, but IMEchE says electric batteries and hydrogen have hogged the limelight, to the neglect of simple systems storing heat. One featured technology at the meeting will be the Heatcube, developed by a Norwegian firm, Kyoto Group. It comes in the form of tanks filled with salt, installed at the site where the heat is needed.

Heatcube’s vertical salt tanks are charged by electricity during periods of low cost. Molten salt is particularly good at holding heat at temperatures up to 500C.

From his office by the Oslo fjord, Kyoto’s chief technology officer, Bjarke Buchbjerg, says: “With all the excitement about battery technology for electric vehicles, people have forgotten about the massive demand for heat for industries that can’t be produced from electrical batteries. Industrial heat is a big deal – we can’t afford to ignore it.”

One of Kyoto’s main shareholders is the Spanish giant Iberdrola. Spain has more than a decade of experience using molten salt to store heat to be discharged at sunset to create steam that makes electricity overnight. Iberdrola’s head of industrial decarbonisation, Fernando Mateo, described energy storage as “one of the major challenges in the energy transition”.

Another young firm using salt is Massachusetts-based Malta, incubated at X, the tech accelerator started by Google (now Alphabet). The firm said the system is intended to complement, rather than compete with other energy storage technologies such as lithium-ion batteries and hydrogen to provide a “missing piece” for the global transition to clean energy. Its main focus is on back-up power generation.

Unlike much sought-after electrical battery components, salt is widely dispersed, easily extracted and able to store heat with minimal degradation or toxic by-products. It is estimated that the salt tanks can be re-charged thousands of times for up to 40 years – at least three times longer than other current storage options.

Dr Robert Barthorpe from Sheffield University, who studies storage options, says: “Molten salt is going to be an important part of the energy mix. It’s a fantastic technology, offering high temperatures at industrial scale.”

Indeed, a California firm, Rondo, claims that its heat battery made from a pile of bricks can store energy at half the cost of green hydrogen and chemical batteries. Its system collects renewable energy and turns it into heat using electrical elements similar to those used in toasters. The firm says these bricks can be heated to 1,500C and are capable of storing energy for days with less than a 1% loss per day.

East Lothian’s Sunamp has received £9.25m in government funding to trial an advanced system in 100 homes across the UK. Its technology relies on storing heat and cold in a phase-change material similar to the gel used in pocket hand-warmers. The firm claims that its patented Plentigrade is four times more efficient at storing heat in tanks of water, allowing it to use a much smaller tank.

In the UK, firms are also turning to compressed air to store energy. Another system, using super-cooled liquid air, was devised by a back-yard inventor in Hertfordshire, Peter Dearman. The company that bought his innovation is Highview Power, which has begun work on a 300MWh facility near Manchester said to store enough power for roughly 50,000 homes for five hours. “We need many different forms of energy storage – and I’m confident liquid air will be one of them,” Dearman says.

Highview has teamed with the energy company Ørsted to investigate how to combine storage with wind energy.

The two companies say the technology can help to reduce curtailment – when wind power owners are paid to switch off their generators to balance the grid – as well as increase productivity, and help the move to a more flexible, resilient zero-carbon grid.

Another firm using compressed air, Cheesecake Energy, employs an electric motor to drive an air compressor which creates high-pressure air, and heat. The battery can then be discharged by running the process in reverse to create electricity.

Other heat battery firms include the Norwegian company EnergyNest, which heats a specially formulated concrete, and Germany’s Lumenion, which stores energy in steel modules up to 650C.

Recent comments by Greg Hands, the former energy and climate change minister, offer hope to the nascent industry. “Driving forward energy storage technologies will be vital in our transition towards cheap, clean and secure renewable energy,” he said.

A 2022 report from the consultancy firm McKinsey highlights the advantages of thermal storage, calculating a price of $65-$100 (£51-£79) a megawatt hour to produce steam heat from hydrogen; $45-$55 for gas with carbon capture and storage; and just $15-$25 for a heat pump with thermal storage.

The report warns of potential challenges for thermal batteries – chiefly because they are little-known by policymakers. Beyond that, there remain potential commercial risks arising from the relative novelty of the industry and the varying maturity of different technologies, given that businesses often have expensive, drawn-out investment cycles.

Its report calls for business leaders to invest in pilots and demonstration plants to create awareness and showcase heat technologies.

Ultimately, with the pressure on to hold down carbon emissions as fast as possible, heat storage start-ups may lose out if the easiest route appears to be to turn to established technologies.