Decarbonizing with a box of hot bricks

Big chemical plants run on heat conveyed by steam, and they run 24 h a day. Most of them operate with the greatest efficiency when they are cranking for months on end without pause. They don’t like to be turned on and off—or even turned up and down.

The output of renewable energy from wind turbines and solar panels, by contrast, swings wildly over the course of a day and is literally as changeable as the weather. That divide is a big part of why the chemical industry’s greenhouse gas emissions are among the hardest to abate. Even as solar and wind become, on average, the cheapest electrons on the grid, the price and availability of the power is too volatile for chemical makers. And, especially in the US, natural gas is still cheaper than electricity as a way to generate heat. So most chemical plants power their boilers with gas or other steady and reliable petroleum sources.

Thermal batteries offer a way to stockpile renewable energy when it’s cheap and use it later. And because they store that energy as heat—which chemical plants and many other industrial facilities need—thermal batteries can beat lithium-ion batteries and other means of energy storage intended to decarbonize heavy industry. The first few installations are running now, and the industry seems poised to expand if the results—and the cost savings—live up to the promise.

The concept of thermal batteries is straightforward: they’re giant, well-insulated toasters filled with bricks. Electricity carries energy to resistive heating elements when renewables are abundant. The bricks, usually some variation of the firebricks used in smelters and furnaces, hold on to it for hours or even days thanks to the insulation. And when renewables are expensive, superheated air carries the heat out to the plant’s boilers or heat exchangers.

The core of the value proposition for thermal batteries is the ability to charge on low-cost renewable electricity, says Juan Camilo Cortés, a senior research associate at Lux Research who studies thermal decarbonization and distributed energy infrastructure. And there’s a lot of spare electricity to sop up at times. Germany, for example, had the equivalent of 24 days of negative electricity prices in 2025, up from 20 the year before.

By leveraging that excess supply, thermal batteries can be a cheaper heat source than natural gas, Cortés says. But it works without subsidies only if the local grid has a lot of renewables and is configured to charge the batteries when wind and solar power are flooding the system.

Thermal batteries have a key advantage over other forms of grid-scale energy storage, Cortés says. They can deliver up to 95% of the energy they take in, a measure known as round-trip efficiency. Traditional batteries and pumped pressure storage methods score between 65% and 85% efficient, according to the US Energy Information Administration. Electrolytic hydrogen production maxes out at around 40%. Ironically, those approaches generate waste heat at every step.

Utilities worldwide are pushing hard toward beefier and more responsive grids, making thermal battery projects bankable in some places today. On Jan. 23, Electrified Thermal Solutions commissioned its first commercial-scale thermal battery at the Southwest Research Institute, a nonprofit contract research organization in San Antonio, Texas, that works on chemical and energy projects from the bench scale up to pilot reactors.

The new unit can store 20 MWh of energy and deliver it in air flowing at up to 1,500 °C. That’s hot enough to run the most thermally demanding reactions, including biomass gasification and methane reformation to get hydrogen. Electrified Thermal sees the Texas unit as a jumping-off point. The company projects that by 2030 it will have installed thermal batteries putting out 2 GW of heat at a cost lower than that provided by natural gas.

Rondo Energy has been even busier. In October, the firm deployed a 100 MWh thermal battery in California that delivers steam to an enhanced oil recovery operation. It’s not a clear win for the climate, but Rondo founder John O’Donnell says the thermal battery allows the site to run on off-grid solar. Without it, Rondo’s customer, Holmes Western Oil, would be powering its steam generator with natural gas to achieve the needed 24 h operation.

In November, Rondo turned on a 33 MWh system at a cement plant in Thailand and announced a deal with Heineken that will see it install a 100 MWh battery at a brewery in Portugal, both ultimately charged by nearby solar farms. O’Donnell says the firm’s batteries deliver heat at up to 1,200 °C.

And last month, Rondo broke ground on a 100 MWh installation at a Covestro chemical plant in Brunsbüttel, Germany, that makes the polyurethane precursor methylene diphenyl diisocyanate. Covestro says that when the battery comes online later this year, it will cut 13,000 metric tons of carbon dioxide per year from the plant’s emissions by shifting 10% of its steam production away from fossil fuel.

Another planned installation, at a plastics recycling plant that Eastman Chemical wants to build in Texas, is on hold after the US Department of Energy rescinded a $375 million grant in June. Rondo and Eastman say they hope to proceed with the project but have not released new timelines or plans to cover the funding gap.

Part of what makes thermal battery makers bullish on rapid expansion is that their supply chains are simple and mature compared with a lot of other clean energy technologies. Most firms don’t rely on rare earth elements or exotic materials. Rondo, for example, uses standard metal alloy heating elements on conventional firebricks made of silica and alumina.

Electrified Thermal and another thermal battery firm, Calectra, mix in elements such as chromium, carbon, and molybdenum to make the bricks electrically conductive, eliminating the need to thread heating elements through the stack. The two companies won’t disclose the exact chemistry they use to make bricks act as resistive heaters, but both describe it as a light modification easily handled by any firebrick supplier.

Rondo’s O’Donnell, whose company bases its thermal batteries on traditional bricks, questions the value of using conductive materials. “Conductive ceramics . . . it’s electrical engineering Disneyland,” he says. Not that it’s all imagination, but it takes a lot of detail-oriented creativity to make it work. “In metals, the resistance goes up with the temperature. In these things, the resistance falls with temperature until you hit some point, and then it goes up the other way. It’s really interesting to manage the electrical engineering, but why would you do it?” he asks.

The answer from Electrified Thermal CEO and cofounder Daniel Stack is that the quirky relationship between temperature and conductivity in conductive ceramics lets them run hotter and operate at voltages that are easier to get from the existing electrical grid.

“We sort of wrote the book on this at MIT,” Stack says. He defended his PhD thesis on resistive firebricks at the Massachusetts Institute of Technology in 2021, and the Electrified Thermal leadership team is heavy with other MIT grads. “Flattening the curves of oxide firebrick materials so that they are in class with existing electric heating systems was the founding achievement of our company,” he says.

What all three firms have in common is that they spend a lot of time on the airflow within the insulated box, which is more chemical engineering than ceramics chemistry. “One of the tricks is to make sure that the brick is heated uniformly,” O’Donnell says, and that it has room to expand and contract. “The stack gets taller and shorter and, horizontally, little gaps open up between them when they’re cold and they just barely touch when they’re hot.”

Stack says the official design lifetime of the units is about 20 years, comparable to other industrial equipment. But with careful control of heat flow, they could last a lot longer. That projected longevity, along with the promise of smoothing out electricity supply-and-demand curves, has helped the leading firms unlock favorable financing.

For example, both Rondo and Electrified Thermal have projects in which a combination of banks, utilities, and governments are putting up the capital to build and install the battery. The chemical or power industry customer just has to make room for it. In most of these deals, the thermal-battery makers will build, own, and operate the system under a model called heat as a service. That setup significantly lowers the risk for the customer and is helping companies ink deals, Stack says.

Executives in the industry are embracing this straightforward model. “We’re building the foundations to make this ordinary,” O’Donnell says. “Because the more ordinary it is, the lower the cost of financing, and therefore the lower-cost source of energy this is going to be.”

Chemical & Engineering News

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