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Why thermal energy storage is poised for a breakout year.
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.
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The nonprofit laid off 36 employees, or 28% of its headcount.
The Trump administration’s funding freeze has hit the leading electrification nonprofit Rewiring America, which announced Thursday that it will be cutting its workforce by 28%, or 36 employees. In a letter to the team, the organization’s cofounder and CEO Ari Matusiak placed the blame squarely on the Trump administration’s attempts to claw back billions in funding allocated through the Greenhouse Gas Reduction Fund.
“The volatility we face is not something we created: it is being directed at us,” Matusiak wrote in his public letter to employees. Along with a group of four other housing, climate, and community organizations, collectively known as Power Forward Communities, Rewiring America was the recipient of a $2 billion GGRF grant last April to help decarbonize American homes.
Now, the future of that funding is being held up in court. GGRF funds have been frozen since mid-February as Lee Zeldin’s Environmental Protection Agency has tried to rescind $20 billion of the program’s $27 billion total funding, an effort that a federal judge blocked in March. While that judge, Tanya S. Chutkan, called the EPA’s actions “arbitrary and capricious,” for now the money remains locked up in a Citibank account. This has wreaked havoc on organizations such as Rewiring America, which structured projects and staffing decisions around the grants.
“Since February, we have been unable to access our competitively and lawfully awarded grant dollars,” Matusiak wrote in a LinkedIn post on Thursday. “We have been the subject of baseless and defamatory attacks. We are facing purposeful volatility designed to prevent us from fulfilling our obligations and from delivering lower energy costs and cheaper electricity to millions of American households across the country.”
Matusiak wrote that while “Rewiring America is not going anywhere,” the organization is planning to address said volatility by tightening its focus on working with states to lower electricity costs, building a digital marketplace for households to access electric upgrades, and courting investment from third parties such as hyperscale cloud service providers, utilities, and manufacturers. Matusiak also said Rewiring America will be restructured “into a tighter formation,” such that it can continue to operate even if the GGRF funding never comes through.
Power Forward Communities is also continuing to fight for its money in court. Right there with it are the Climate United Fund and the Coalition for Green Capital, which were awarded nearly $7 billion and $5 billion, respectively, through the GGRF.
What specific teams within Rewiring America are being hit by these layoffs isn’t yet clear, though presumably everyone let go has already been notified. As the announcement went live Thursday afternoon, it stated that employees “will receive an email within the next few minutes informing you of whether your role has been impacted.”
“These are volatile and challenging times,” Matusiak wrote on LinkedIn. “It remains on all of us to create a better world we can all share. More so than ever.”
A battle ostensibly over endangered shrimp in Kentucky
A national park is fighting a large-scale solar farm over potential impacts to an endangered shrimp – what appears to be the first real instance of a federal entity fighting a solar project under the Trump administration.
At issue is Geenex Solar’s 100-megawatt Wood Duck solar project in Barren County, Kentucky, which would be sited in the watershed of Mammoth Cave National Park. In a letter sent to Kentucky power regulators in April, park superintendent Barclay Trimble claimed the National Park Service is opposing the project because Geenex did not sufficiently answer questions about “irreversible harm” it could potentially pose to an endangered shrimp that lives in “cave streams fed by surface water from this solar project.”
Trimble wrote these frustrations boiled after “multiple attempts to have a dialogue” with Geenex “over the past several months” about whether battery storage would exist at the site, what sorts of batteries would be used, and to what extent leak prevention would be considered in development of the Wood Duck project.
“The NPS is choosing to speak out in opposition of this project and requesting the board to consider environmental protection of these endangered species when debating the merits of this project,” stated the letter. “We look forward to working with the Board to ensure clean water in our national park for the safety of protection of endangered species.”
On first blush, this letter looks like normal government environmental stewardship. It’s true the cave shrimp’s population decline is likely the result of pollution into these streams, according to NPS data. And it was written by career officials at the National Park Service, not political personnel.
But there’s a few things that are odd about this situation and there’s reason to believe this may be the start of a shift in federal policy direction towards a more critical view of solar energy’s environmental impacts.
First off, Geenex has told local media that batteries are not part of the project and that “several voicemails have been exchanged” between the company and representatives of the national park, a sign that the company and the park have not directly spoken on this matter. That’s nothing like the sort of communication breakdown described in the letter. Then there’s a few things about this letter that ring strange, including the fact Fish and Wildlife Service – not the Park Service – ordinarily weighs in on endangered species impacts, and there’s a contradiction in referencing the Endangered Species Act at a time when the Trump administration is trying to significantly pare back application of the statute in the name of a faster permitting process. All of this reminds me of the Trump administration’s attempts to supposedly protect endangered whales by stopping offshore wind projects.
I don’t know whether this solar farm’s construction will indeed impact wildlife in the surrounding area. Perhaps it may. But the letter strikes me as fascinating regardless, given the myriad other ways federal agencies – including the Park Service – are standing down from stringent environmental protection enforcement under Trump 2.0.
Notably, I reviewed the other public comments filed against the project and they cite a litany of other reasons – but also state that because the county itself has no local zoning ordinance, there’s no way for local residents or municipalities opposed to the project to really stop it. Heatmap Pro predicts that local residents would be particularly sensitive to projects taking up farmland and — you guessed it — harming wildlife.
Barren County is in the process of developing a restrictive ordinance in the wake of this project, but it won’t apply to Wood Duck. So opponents’ best shot at stopping this project – which will otherwise be online as soon as next year – might be relying on the Park Service to intervene.
And more on the week’s most important conflicts around renewable energy.
1. Dukes County, Massachusetts – The Supreme Court for the second time declined to take up a legal challenge to the Vineyard Wind offshore project, indicating that anti-wind activists' efforts to go directly to the high court have run aground.
2. Brooklyn/Staten Island, New York – The battery backlash in the NYC boroughs is getting louder – and stranger – by the day.
3. Baltimore County, Maryland – It’s Ben Carson vs. the farmer near Baltimore, as a solar project proposed on the former Housing and Urban Development secretary’s land is coming under fire from his neighbors.
4. Mecklenburg County, Virginia – Landowners in this part of Virginia have reportedly received fake “good neighbor agreement” letters claiming to be from solar developer Longroad Energy, offering large sums of cash to people neighboring the potential project.
5. York County, South Carolina – Silfab Solar is now in a bitter public brawl with researchers at the University of South Carolina after they released a report claiming that a proposed solar manufacturing plant poses a significant public risk in the event of a chemical emissions release.
6. Jefferson Davis County, Mississippi – Apex Clean Energy’s Bluestone Solar project was just approved by the Mississippi Public Service Commission with no objections against the project.
7. Plaquemine Parish, Louisiana – NextEra’s Coastal Prairie solar project got an earful from locals in this parish that sits within the Baton Rouge metro area, indicating little has changed since the project was first proposed two years ago.
8. Huntington County, Indiana – Well it turns out Heatmap’s Most At-Risk Projects of the Energy Transition has been right again: the Paddlefish solar project has now been indefinitely blocked by this county under a new moratorium on the project area in tandem with a new restrictive land use ordinance on solar development overall.
9. Albany County, Wyoming – The Rail Tie wind farm is back in the news again, as county regulators say landowners feel misled by Repsol, the project’s developer.
10. Klickitat County, Washington – Cypress Creek Renewables is on a lucky streak with a solar project near Goldendale, Washington, getting to bypass local opposition from the nearby Yakama Nation.
11. Pinal County, Arizona – A large utility-scale NextEra solar farm has been rejected by this county’s Board of Supervisors.