He stores cheap renewable electricity inside blocks of volcanic rock, then hands it back as steam. The rock does not care what time of day the sun shone.
At three or four in the morning, most people chase sleep. James Macnaghten chased a question instead: could you marry aluminium, which moves heat brilliantly, to rock, which costs almost nothing? That bedroom hunch is now sitting in steel cabinets in Hampshire, soaking up electricity when it is cheap and abundant and giving it back as industrial steam when a factory needs it.
Today he is co-founder and CEO of Caldera, the company built around that idea. Its “Warmstone” heat cell packs porous volcanic rock and recycled aluminium into a core, wraps it in a proprietary vacuum insulation that works like a giant flask, and holds the heat at temperatures up to 300°C. When a brewery, a dairy, a distillery or a hospital boiler room calls for heat, the stone delivers hot water or steam at up to 200°C and around ten bar of pressure. No gas flame. No carbon. Just stored sunshine, released on demand.
The pitch is disarmingly practical, and that is the point. Macnaghten is not interested in the heroic, hard-to-crack corners of decarbonisation. He is interested in the enormous, boring middle. “We are interested in everything under 200 degrees Celsius,” he says, “which is between 30% and 40% of all industrial heat used worldwide; it produces more carbon emissions than all cars in the world.” Read that twice. The warm water that pasteurises milk and washes bottles pollutes more than every car on the planet, and almost nobody talks about it.
His frustration with the conversation is genuine. “Why are we tackling hard-to-decarbonise problems when we have solutions for something like 50% of all industrial heat today?” he asks. It is the engineer’s instinct to fix the solvable thing first, and it runs through everything Caldera does.
Macnaghten did not arrive at heat batteries by accident. He trained as a mechanical engineer at the University of Cambridge between 1988 and 1991, and he has spent the years since turning thermodynamics into businesses. For roughly a decade he was CEO of Isentropic, a company developing large-scale thermo-mechanical electricity storage - a way of squeezing and expanding gas to store grid power as heat and cold. He led a team of engineers chasing a technology that, if it worked at scale, could smooth out the entire electricity system.
That decade left him with two things: a fistful of patents and a hard-won conviction. “Guy and I have worked together in energy storage for years,” he says of his Caldera co-founder, Guy Winstanley. “I realised that storage has to be phenomenally cheap to be worth putting in.” It is a deceptively simple lesson. Plenty of clever storage exists. Very little of it is cheap enough that a factory owner will buy it without being forced to. Volcanic rock and scrap aluminium are, above all, cheap.
He is, by reputation and output, a prolific inventor - named as co-inventor or sole inventor across more than twenty patent families in thermal and electrical storage. But he wears it lightly. The motivation he keeps returning to is not the cleverness of the device; it is the dent it might make. “I want us to have an impact,” he says. “Everyone in the company wants to have an impact.”
There is a second, stranger line on his CV, and it is the one that makes people lean in. Macnaghten built, and still owns, Scudamore’s Punting Co - Cambridge’s largest tourist business. Yes, the punts: the flat-bottomed boats poled along the Cam past the backs of the colleges. The same man who designs vacuum-insulated heat cells for breweries also runs the city’s most photographed boat fleet. One business moves tourists slowly down a river; the other moves the energy transition quietly forward. He is comfortable holding both.
It tells you something about how he thinks. A punting company is a real business with real customers, weather, seasons and margins. It is not a slide deck. Macnaghten has spent more than twenty-five years running businesses that have to actually work, and he brings that same unsentimental eye to climate technology. The question is never just “is it elegant?” It is “will someone pay for it, and does it pencil out?”
For a long time, heat batteries lived in the overlooked corner of the clean-energy story - less glamorous than lithium, harder to explain than solar panels. 2025 changed Caldera’s footing. In March, the German industrial-engineering giant GEA Group took a stake in the company as part of a Series A, an investment reported at around €12 million. For a business selling into breweries, distilleries, dairies, food and pharmaceutical plants, GEA is not just money; it is a route straight into the factories that need the technology.
“The investment from GEA is a game-changing opportunity for our company and a strong endorsement of our technology,” Macnaghten said at the time. Partnering with GEA, he explained, would let Caldera deploy its storage boilers across exactly those heat-hungry industries “with the ultimate goal of reducing carbon emissions for the benefit of all.”
Then came the order that turns a prototype into a product: a 10 MWh electric Storage Boiler bound for an NHS hospital. A hospital is a perfect proving ground - it needs reliable heat around the clock, it cannot tolerate failure, and it answers to taxpayers watching every pound of energy spend. If the stone keeps the wards warm, the argument makes itself.
Ask him where this goes and the answer is geographic and concrete. “We will be selling all over Europe,” he says of the years ahead. “We will have units in North America, or be looking to open in North America.” No moonshot rhetoric, no promises of saving the world by Tuesday - just a map and a plan to put cabinets of warm rock next to the boilers they are meant to replace.
And when the schedule allows, he points his bike at the South Downs and rides. The hills do not store heat, but they clear the head - useful for a man whose best idea, by his own account, arrived in the small hours of the night. The rock has been holding onto that idea ever since.
In bed at night, at three or four in the morning, I had this really weird idea: could you combine aluminium, with good conductivity, with rock, which is very low cost?— James Macnaghten, on the origin of Caldera’s heat cell
When renewable electricity is abundant and prices are low, the boiler heats a core of porous volcanic rock and recycled aluminium.
A proprietary vacuum insulation - the principle behind a thermos - holds the heat for long stretches with little loss.
When a factory or hospital needs it, the stone gives back hot water or steam up to 200°C and around ten bar. No flame, no carbon.
Figures as described by James Macnaghten in interview. Bars illustrative.
“Storage has to be phenomenally cheap to be worth putting in.”
“I want us to have an impact. Everyone in the company wants to have an impact.”
“Why are we tackling hard-to-decarbonise problems when we have solutions for something like 50% of all industrial heat today?”
“We will be selling all over Europe. We will have units in North America, or be looking to open in North America.”
The heat cell stores energy in volcanic rock and recycled aluminium - low-cost materials chosen precisely because they are cheap.
His best idea arrived at 3 or 4am. The patent stack came later.
He owns Scudamore’s Punting Co, the largest tourist business in Cambridge, alongside running a climate-tech firm.
The heat Caldera targets - everything under 200°C - emits more carbon than all the world’s cars.