Cheap clean energy could unleash the power of thermal storage

A growing crop of startups plans to use heat and rocks to tackle some of the hardest decarbonization problems, from chemical production to long-term grid storage.
By Eric Wesoff

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A large white metal box with yellow stairs positioned at an outdoor industrial site
Antora's thermal battery (Antora)

Rocks and hotness have existed for billions of years, but it’s only now that the two can be used to help the world decarbonize — and it’s all because the insanely low cost of solar and wind power has made thermal storage economically possible.

In essential industries such as steel, cement and chemicals production, the massive amount of energy needed to heat stuff up makes up a big part of their emissions footprints and production costs. Heavy industry accounts for over one-quarter of global emissions, and nearly 17 percent of all energy used by industry in the U.S. is for heating things and generating steam.

And for another essential industry to truly decarbonize — the power sector — 24/7 clean energy is needed; renewable energy credits and intermittent renewables paired with short-duration batteries won’t cut it.

Startups — nearly 30 of them, by Canary’s count — are now attempting to solve these climate problems by harnessing the power of thermal storage. They plan to use surplus renewable electricity to heat up rocks and other materials with high thermal mass to store cheap clean energy for later use. Some startups plan to dispatch this energy as heat for industrial processes, while others plan to feed it back into the grid on demand as clean electricity. An enterprising few plan to do both.

Investors ranging from Bill Gates to the world’s largest mining concerns have in recent years bet hundreds of millions of dollars that at least one company in this crowded field will be able to scale up a business based around using hot rocks to curb carbon emissions.

So why is now the right time for this up-and-coming technology?

What has fundamentally changed is the price of renewables. You have immense quantities and immense potential for very cheap renewable energy coming from wind and solar,” said Andrew Ponec, co-founder and CEO of thermal storage startup Antora, in an interview with Canary Media at his company’s newly built manufacturing facility in San Jose.

Antora is fresh off a funding round last week, during which it collected $150 million from an elite group of corporate and venture investors. The startup will use the new injection of funds to commercialize its thermal storage design and scale production at its San Jose facility and beyond. Rondo, another thermal storage startup, collected $60 million from a cadre of premier investors and industrialists late last year, and it is also using that money to expand its production.

We’re at a moment where people are figuring out that this is going to be one of the big hammers [to decarbonize industry]. It works on smaller facilities, [and] it works on larger ones,” Rondo CEO John O’Donnell told Canary in October.

An overheated field 

Decarbonizing the U.S. manufacturing complex is one of the heaviest climate lifts, but it’s also a trillion-dollar business opportunity that’s attracted cash, entrepreneurs, investors, industry and government.

Different industries have different heat requirements. Processing of food, textile, paper and chemicals runs up to several hundred degrees Celsius, a temperature that can be reached with fossil fuels, an electric furnace or a thermal storage medium like molten salt.

But processes such as cement, glass and petrochemical production and smelting steel require much higher temperatures, extending up to 1,500°C or more. Those temperatures are readily produced by burning fossil fuels, but they’re harder to reach with electrified furnaces. Getting to those blistering heights with renewables-fueled thermal storage could be the key that unlocks carbon-free industrial processes, though other pathways based around solutions like clean hydrogen are being explored too.

Just as every industry has its own heat requirements, each thermal storage company has its own preferred heat-trapping material. Some use salts, some use metals — and plenty use rocks. The table below shows the wide array of startups and techniques in this space. 

Rondo, for its part, is storing heat in a simple way — in bricks. It’s not a new idea. The steel industry has been using bricks for thermal storage in blast furnaces as a coal-saving technology for 200 years, according to O’Donnell, who called it waste-heat storage at its finest.”

We use resistive heating elements with a new structure that let us directly heat brick with radiant heat — the way your toaster heats bread, the way the sun heats the earth.”

Rondo pulls heat out of the bricks with forced air that flows into a kiln and then to a boiler to make steam that drives a plant, for example. We’re running our first pilot unit right now at a facility that makes ethanol. This is the first part of their journey to zero carbon.”

Instead of brick, Antora heats graphite blocks to a white-hot glow, reaching up to 2,000°C. The blocks are housed in a well-insulated box about the size of a shipping container.

When a shutter on the wall of Antora’s furnace is opened, an intense beam of heat is emitted from the glowing graphite. That beam can be directed toward a boiler or heat-transfer plumbing, replacing the heat from fossil-fuel combustion.

The startup has another trick up its sleeve: It’s also using special photovoltaic panels, thermophotovoltaics, or TPV, to turn the heat energy into electricity. Antora’s first product will store and discharge heat; its next design will be capable of providing heat and electricity, according to the company.

Antora has deployed a 5-megawatt-hour storage prototype at Wellhead Electric Company’s facility near Fresno, California.

While Rondo is selling heat and Antora is selling both heat and electricity, newly unstealthed Fourth Power is focused exclusively on producing electricity. It’s not targeting industrial heat like many of its competitors but instead aims to solve the notoriously difficult task of storing clean energy for the grid over daily or even seasonal time frames. Like Antora, it uses super-heated graphite blocks as a storage medium.

David Roberts, Canary editor-at-large and podcaster of renown, has said he has a penchant for startups that use renewable energy to heat up a box of rocks.” Roberts recently interviewed Asegun Henry, founder and chief technology officer at Fourth Power, on an episode of his Volts podcast.

Now equipped with $19 million in Series A funding, the startup uses molten tin as a heat transfer fluid, along with pipes and pumps made from carbon, a noncorrosive, nonreactive combination. A radiator heats up the liquid tin and circulates the molten metal around and through large blocks of graphite until the rocks are blazing hot.

Fourth’s process then focuses the extremely intense light emitting from the glowing graphite onto thermophotovoltaic cells that can reach efficiencies of up to 40 percent, according to a 2022 paper published in Nature by Fourth CTO Henry in collaboration with researchers from MIT and the National Renewable Energy Laboratory. Henry said the company is targeting efficiencies of up to 50 percent. (For comparison, conventional commercial solar cells, typically made from crystalline silicon, max out at about 24 percent efficiency. The TPV cells used by Antora and Fourth are made of less readily abundant materials and are more complex and costly to make.)

Henry told Roberts that he sees TPV’s high conversion efficiency as the key to replacing fossil gas turbines with clean, dispatchable power. He contends that hitting 40 percent efficiency with the TPV cells is of immense significance because this is the first time a solid-state heat engine” has performed better than a typical steam turbine [with an]…efficiency [around] 33 percent.”

Fourth Power is building a demonstration project north of Boston with a capacity of around 10 megawatt-hours.

Antora, Rondo, Fourth Power and their many competitors face a long and costly road ahead. Even with all their promise and funding, they will have to figure out how to cross the climatetech valley of death — the chasm between running a modest pilot plant and scaling up to achieve commercial viability. The financial stakes are high, with more than $800 million invested in this sector over the last few years, but the planetary stakes of cracking the code on difficult decarbonization problems are far higher.

Eric Wesoff is editorial director at Canary Media.