The Coolest Thing in Climate Tech is a Super Hot Rock

One of the oldest ways to store up energy is in hot rocks. Egyptians built adobe homes millennia ago that absorbed heat during the day and released it at night, and wood-fired ovens with bricks that radiate residual heat have been around since the Middle Ages.

Now, this ancient form of heating is poised for a breakout year as one of the hottest things in climate tech: thermal batteries. These aren’t the kinds of batteries you’d find in a laptop or electric vehicle. Instead, these stationary, shipping container-sized units can provide the high temperatures necessary to power hard-to-decarbonize industrial processes like smelting or chemical manufacturing. And thanks to the changing economics of clean energy and a generous tax credit in Biden’s Inflation Reduction Act, investors are increasingly bullish about the technology, helping Silicon Valley startups Antora Energy and Rondo Energy dramatically scale up production with new gigafactories.

The underlying technology is fairly basic. Using essentially the same technology as a toaster, electricity from renewable energy is converted into heat and then stored in thermally conductive rocks or bricks. That heat is then delivered directly as hot air or steam to the industrial facilities that the stationary batteries are sited on. Rondo says it can supply continuous heat at full capacity — that’s over 1,000° Celsius — for 16 to 18 hours, and Antora’s system is rated at 25 hours, helping fill the gaps when sun and wind resources are scarce.

Rondo’s thermal battery at an ethanol plant in California.Courtesy of Rondo Energy.

The climate benefits of this process are clear — and potentially huge. Heat alone comprises half of the world’s total energy consumption, and about 10% of global CO2 emissions come from burning fossil fuels to generate the high temperatures necessary for industrial processes like steel and cement production, chemicals manufacturing, and minerals smelting and refining. These industries are notoriously hard to decarbonize because burning gas or coal has been much cheaper than using electricity to generate high heat.

That’s also why we haven’t traditionally heard a lot about thermal batteries. Before renewables became ubiquitous, the tech just wouldn’t have been very clean or very cheap.

But thanks to the rapidly falling cost of wind and solar, its economics are looking increasingly promising. “There’s this glut of cheap, clean power that is just waiting to be used,” Justin Briggs, Antora’s co-founder and COO, told me. “It’s just going to waste in a lot of cases already.”

John O’Donnell, the co-founder and CEO of Rondo, concurred.“This industrial decarbonization is going to start out absolutely absorbing those negative and zero prices,” he told me. “But it is also going to drive massive new construction of new renewables specifically for its own purpose.”

Of course thermal batteries aren’t the only technology trying to solve industrial heat emissions. Concentrating solar thermal power systems can store the sun’s heat in molten salts, carbon capture and storage systems can pull the emissions from natural gas combustion at the source, and green hydrogen can be combusted for heat delivery.

Indeed, the same forces making thermal energy more attractive are also benefiting green hydrogen in particular. Cheap renewables and lucrative hydrogen subsidies in the IRA mean green hydrogen is also poised to rapidly fall in price. But proponents of thermal batteries argue their technology is much more efficient.

Electrical resistance heating (i.e. turning electricity into heat like a toaster) is already a 100% efficient process. And after storing that heat in rocks for hours or days, you still can get over 90% of it back out. But producing green hydrogen through electrolysis and subsequently combusting it for heat is generally only about 50-66% efficient overall, says Nathan Iyer, a senior associate at the think tank RMI. Although emerging electrolyzer technologies like solid oxide fuel cells can push efficiencies over 80%, in part by recycling waste heat, many green hydrogen production methods could require around 1.5 to two times the amount of renewable electricity as thermal batteries to generate the same amount of heat.

“Pretty much all of the major models are saying thermal batteries are winning when they run all of their optimizations,” Iyer said. “They’re finding a huge chunk of industrial heat is unlocked by these thermal batteries.”

However, when it comes to the most heat-intensive industries, such as steel and cement production, combusting green hydrogen directly where it’s needed could prove much easier than generating and transporting the heat from thermal batteries. As Iyer told me, “At a certain level of heat, the materials that can actually handle the heat and move the heat around the facility are very, very rare.”

Iyer says these challenges begin around 600° or 700° Celsius. But the lion’s share of industrial processes take place below this temperature range, for use cases that thermal batteries appear well-equipped to handle.

And now, the gigafactories are on their way. Rondo has partnered with one of its investors, Thailand-based Siam Cement Group, to scale production of its heat battery from 2.4 gigawatt-hours per year to 90 GWh per year, which will equal about 200-300 battery units. This expanded facility would be the largest battery manufacturing plant in the world today — about 2.5 times the size of Tesla’s Gigafactory in Nevada.

Rondo, which has raised $82 million to date, says it can scale rapidly because its tech is already so well understood. It relies on the same type of refractory brick that’s found in Cowper stoves, a centuries old technology used to recycle heat from blast furnaces.

In Rondo’s case, renewable electricity is used to heat the bricks instead. Then, air is blown through the bricks and superheated to over 1,000° Celsius before being delivered to the end customer as either heat through a short high-temperature duct or as steam through a standard boiler tube.

“We’re using exactly the same heating element material that’s in your toaster, exactly the same brick material that’s in all those steel mills, exactly the same boiler design and boiler materials so that we have as little to prove as possible,” O’Donnell says.

Currently, Rondo operates one small, 2 megawatt-hour commercial facility at a Calgren ethanol plant in California. The company hopes to expand its U.S. footprint, something the IRA will help catalyze. Last month’s guidelines from the IRS clarify that thermal batteries are eligible for a $45 per kilowatt-hour tax credit, which will help them compete with cheap natural gas in the U.S.

Antora is already planning to produce batteries domestically, recently launching its new manufacturing facility in San Jose, California. The company has raised $80 million to date, and operates a pilot plant in Fresno, California. Similar to Rondo, Antora’s tech relies on common materials, in this case low-grade carbon blocks. “It’s an extremely low-cost material. It’s produced at vast scales already,” says Briggs.

Antora’s carbon blocks.Courtesy of Antora Energy

When heated with renewable electricity, these blocks emit an intense glow. Much like the sun, that thermal glow can then be released as a beam of 1,500° Celsius heat and light through a shutter on the box.

“And you can do one of two things with that beam of light. One, you can let that deliver thermal energy to an industrial process,” says Briggs. Or Antora’s specialized thermophotovoltaic panels can convert that hot light back into electricity for a variety of end uses.

It’s all very promising, but ultimately unproven at scale, and the companies wouldn’t disclose early customers or projects. But they have some big names behind them. Both Antora and Rondo are backed by the Bill Gates-funded Breakthrough Energy Ventures. Antora also receives funding from Lowercarbon Capital, Shell Ventures, and BHP Ventures, indicating that the oil, gas, petrochemical, and mining industries are taking note.

Along with funding from Energy Impact Partners, Rondo has a plethora of industry backers too, including Siam Cement Group, TITAN Cement Group, mining giant Rio Tinto, Microsoft’s Climate Innovation Fund, Saudi chemicals company SABIC, and oil company Saudi Aramco.

“The investors that just joined us have giant needs,” O’Donnell says of the company’s decision to massively ramp up manufacturing. “Rio Tinto has announced 50% decarbonization by 2030. Microsoft is buying 24-hour time-matched energy in all kinds of places. SABIC and Aramco have enormous steam needs that they want to decarbonize.”

Primary uses of this tech will likely include chemical manufacturing, mineral refining, food processing and paper and biofuel production. Industries like these, which require heat below 1,000° Celsius (and often much less), account for 68% of all industrial emissions. While steel and cement production are two of industry’s biggest emitters, their heat needs can exceed 1,500° Celsius, temperatures that Rondo and Antora admit are more technically challenging to achieve.

In any case, 2024 is the year when hot rocks could start making a dent in decarbonization. The IRA’s tax credits mean this emergent tech could become competitive in more markets, beyond areas with excess renewable power or substantial carbon taxes. This is the year that Antora says they’ll begin mass production, and Rondo’s first commercial projects are expected to come online.

As O’Donnell says, “This is not 10 years away. It’s not five years away. It’s right now.”

Editor’s note: This article was updated after publication to account for emerging electrolyzer technologies.