Peter Mares, Science and Technology correspondant for The Monthly visits the Caldera factory in Fareham, UK and talks to James Macnaghten, co-founder and CEO.
“Put your hand on that,” urges James Macnaghten, indicating a fat metal cylinder about one metre wide and 1.7 metres tall. Towards the top of its steel casing I feel the faintest warmth; lower down it’s stone cold. “The material inside is 400 degrees Celsius,” he says with a grin.
Macnaghten is an ebullient inventor and entrepreneur with more than 20 patents to his name. I’ve come to his factory in an industrial estate 100 kilometres south-west of London to find out about his most promising project: a thermal battery. Batteries are crucial to Australia’s energy transition and we generally think of them as devices for storing and releasing energy as electricity. Thermal batteries, however, store and release energy as heat. And heat has many uses.
“Half of all energy use in the world is heat, and half of that heat is used by industry,” explains Macnaghten. Australia broadly conforms to this picture: industry consumes 44 per cent of national energy production, and just over half of it is used for “process heat”. Most of that heat comes from burning gas and coal. Wind and solar generate cheaper power, but their variability is a problem for industries that need heat 24-hours a day, which is why we’re frequently told that we still need gas and can’t transition to renewable energy without it.
This is where thermal batteries come in: they have the potential to store intermittent solar and wind power, and deliver non-stop industrial heat. Australia has surplus renewable electricity in the middle of the day when solar generation peaks and household use dips. If thermal batteries captured cheap excess power, stored it as heat and made it constantly available for industrial uses, that could liberate businesses from the vagaries of unpredictable gas pricing and slash carbon pollution at the same time. “On the scale of what we need to tackle global warming and reduce emissions, industrial heat is the easy win,” says Macnaghten.
Caldera, the firm Macnaghten founded with business partner and fellow engineer Guy Winstanley, is targeting a significant subset of the industrial heat market: steam and pressurised hot water at temperatures between 100 and 200 degrees Celsius. Supposedly, the steam age ended in the 20th century, but Macnaghten says it never went away: “Steam is still used to make almost everything you eat and drink.” Other industries that use steam and pressurised hot water include pharmaceuticals, paper and pulp, and aluminium refining.
The steel cylinder I touched is a 100kWh prototype domestic thermal battery, which provides heat and hot water to the factory offices. (A kWh or kilowatt hour is a measure of energy use. Over 10 hours, a 100-watt light bulb would consume one kWh of electricity.) Macnaghten has another battery the same size at home, as does Winstanley. All three batteries have been running safely and reliably for five years.
Macnaghten’s original vision was to manufacture cost-effective, low-carbon domestic heating systems as an alternative to gas boilers, which must be phased out for Britain to reach its goal of net zero emissions by 2050. But the proposition didn’t stack up, because the battery was too bulky and too heavy for most homes (it weighs 1.7 tonnes). Electric heat pumps have, in any case, proven to be a more cost-effective replacement for domestic gas.
Rather than be discouraged, Macnaghten started thinking bigger. The prototype had proved the feasibility of his overall concept, and he knew heat pumps were not effective at meeting industry’s needs to rapidly heat water to very high temperatures. Something small also loses heat faster than something large. So, as his prototype battery stays hot for days at a time, Macnaghten figured a bigger version could store heat even longer, before needing to be recharged.
In 2021, Caldera crowdfunded £1.5 million ($3 million) via an investment website to develop the idea. The following year, it won £4.3 million from the UK Department for Energy Security and Net Zero in a competition to find innovative ways to help industry to switch from high- to low-carbon fuels. Caldera secured the funds to demonstrate and test a four megawatt-hour industrial storage boiler – 40 times the capacity of Macnaghten’s domestic prototype. The spike in gas prices after Russia’s invasion of Ukraine gave the project extra impetus. Caldera’s industrial scale system began testing last year and has hit all its targets.
On Caldera’s factory floor, we weave our way between hoists, forklifts, and wooden pallets stacked high with ingots of aluminium, mostly from melted down motors. “There are lots of leftover engine blocks, and there will be even more as we transition from petrol to electric vehicles,” says Macnaghten. “We are a sink for the lowest grade of aluminium.”
Residual engine grease and random steel bolts prevent this scrap aluminium being used for high grade purposes, but for Caldera the impurities don’t matter. The ingots are cast in a furnace together with crushed rock to create a composite material that sits inside the thermal battery, which is why Macnaghten’s prototype is so heavy.
The brainwave of mixing aluminium and rock came to Macnaghten in the middle of the night when he was trying to think of a material that would be solid, conductive, readily available and cheap. When he tested the idea the next morning, the initial pour was a dud. “The rocks floated on the surface of the aluminium like popcorn,” he recalls with a laugh.
Trial and error proved that volcanic basalt had the right density to spread evenly through molten aluminium, like gravel mixed through concrete. But developing a storage medium was only one challenge. Many things can hold heat, and other thermal batteries use rock, metal, concrete, sand, salt or water. “The problem is not finding the right material to store heat,” Macnaghten says. “The problem is getting the heat in and out easily, and keeping it in there without it leaking away.”
Step one, getting the heat in, is relatively simple. An electric heating element runs through the battery and the aluminium distributes the heat through the core, rapidly heating it to 500 degrees Celsius. If the power used to heat the element comes from renewable sources, emissions from coal and gas are avoided. The design keeps the element itself cool, so Macnaghten says it will last for decades.
Step two, getting the heat out again, requires more ingenuity. Macnaghten points to a tiny pump on the top of his prototype battery, which he repurposed from an espresso machine. The pump injects a small amount of water into the tank – just enough to brew two short blacks – where the hot rocks turn it into steam. The steam is piped into a heat exchanger to warm a much larger body of liquid. The same principle applies to his four MWh industrial scale battery, although in this case the steam is also under very high pressure. Macnaghten guides me to a stainless-steel tank that resembles an enormous tea urn. It holds 15,000 litres of water, about 50 times the capacity of a domestic hot-water system. Macnaghten says the four MWh battery can boil that volume of water in under an hour.
Step three, keeping the heat in, uses familiar technology. The battery is encased in a vacuum flask, like a giant thermos, made from high-grade steel. Caldera’s industrial-scale thermal battery sits in the factory yard. Compared to the prototype indoors it’s giant, standing seven metres tall. But it’s designed to be assembled on the site, with wedges of the aluminium-basalt composite stacked into a column. The vacuum casing is then craned over the top and inverted over the battery core. The component parts of the battery can all fit on standard trucks, so they can be transported without permits or escorts.
Macnaghten says Caldera’s system is easy to retrofit to replace conventional gas boilers, and much safer to operate. Analysis from energy consultants found it could cut energy bills almost in half and pay for itself in under six years.
The inventor says Caldera’s batteries can fully charge in just four hours and can be arrayed in series to store up to 100MWh of energy. That’s almost as much capacity as was originally installed in the Hornsdale Power Reserve, the “big battery” that the South Australian government bought from Elon Musk’s Tesla in 2017. At the time, it was the largest lithium-ion battery in the world. Since then, it has been expanded, and stabilises a state grid in which more than 70 per cent of electricity is generated from renewable sources. Malcolm Turnbull’s much vaunted and much delayed Snowy 2.0 will eventually play a similar role on the east coast using pumped hydro, alongside newer projects such as the 1000 MWh Stoney Creek Battery Energy Storage System near Narrabri. The recent federal election campaign featured incentives to install domestic batteries, which stack up particularly well for households with rooftop panels pumping out excess solar power in the middle of the day.
But thermal batteries providing on-site industrial heat as an alternative to gas are not really on the public radar. At least not yet. Macnaghten thinks it would be easy to manufacture Caldera’s batteries anywhere in the world, including in Australia. German engineering giant GEA sees the global potential and, not long after my visit, it announced a €12 million ($21 million) investment in Caldera, saying its system offers a scalable, economically viable alternative to fossil fuel boilers, and is ideally suited to “decarbonize fluctuating industrial heat demand between 100 and 200°C”.
The world is in trouble, and we are not meeting the Paris climate targets. Macnaghten is a practical man looking for practical, profitable solutions. “We should do everything that makes money today and do it quickly,” he says. “If you want to decarbonise, tackle industrial heat first.”