A Silicon Valley startup is developing a hot rock battery that could be the next big thing in energy storage — and it’s coming to Europe.
The giant brick toaster is the brainchild of San Francisco-based Rondo Energy. The startup believes it could be a cheap, scalable way to decarbonise industries making everything from steel to soap.
Rondo has already raised over $100mn from investors including Microsoft, mining conglomerate Rio Tinto, and Saudi Arabia’s oil and gas giant Aramco.
Speaking of Microsoft, Bill Gates’ Breakthrough Energy Ventures was Rondo’s first major investor. And this week, the climate tech fund opened its coffers once more.
Rondo has secured €75mn from Breakthrough Energy Catalyst, and the European Investment Bank.
Founded in 2020, Rondo has already proven its technology in the US and Thailand. Armed with the fresh funds, it looks to take on the European market. In Europe, “we see very strong tailwinds to the adoption of our technology,” said Eric Trusiewicz, CEO of Rondo Energy.
The company is planning to build three batteries at three very different factories in Denmark, Germany, and Portugal.
In Denmark, the hot rock battery will be used to produce cleaner biogas. In Germany, it will power a Covestro chemical plant that produces polymers. In Portugal, it will power a yet-to-be-announced food and beverage facility.
How does the hot rock battery work?
Rondo’s hot rock battery uses renewable energy to power a network of electrical resistance heaters. These heat a stack of specially-designed bricks to a cosy 1,500°C.
Rondo developed the rocks alongside the Siam Cement Group in Thailand. The partners designed the bricks to be just the right size, shape, and density to maximise their ability to store and release energy.
The bricks are encased in an iron box that is so well insulated, they only lose around 1% of their heat per day. This means the device can store excess energy and release it on demand.
AI automates the exact temperature of the battery. This patented machine learning algorithm also controls when the battery releases energy — to provide the right kind of heat at the right time. District heating, for example, will require much lower temperatures than a steel-making plant. The machine delivers energy either as superheated steam or hot air.
Industrial manufacturing uses over a third of the world’s energy. A lot of this power comes in the form of heat produced by fossil fuel-powered industrial boilers. Rondo believes that its hot rock batteries can replace these boilers in 90% of applications.
The next hot energy storage tech?
For Europe, the energy storage system could be a triple win, says Ann Mettler, VP of Breakthrough Energy Europe.
“Europe urgently needs to replace imported gas with domestic renewable energy,” she said. “Repowering our industries is a matter of climate security, economic security, and national security.”
Rondo isn’t the only company competing to solve the challenge of clean heat for heavy industry. Some are working on using hydrogen or biogas to replace natural gas in boilers. Others are using mega-scale heat pumps. But these technologies all face the same problem: they’re more expensive than the status quo.
While Rondo’s idea is rather simple, that could be its greatest strength. The battery doesn’t require any technological breakthroughs or rare and expensive materials. The system is modular, and factory operators can easily plug it into their existing energy system. According to the startup, it also saves factories money.
The three projects announced this week follow a number of big deals for Rondo, including one with a plastic recycling plant in Texas and another with a whiskey distillery in Kentucky. Fast fashion giant H&M recently backed Rondo, and the company has also secured a partnership with renewable energy giant EDP.
But its most ambitious plan of all is to build the world’s biggest battery plant in Thailand for Siam Cement. The factory would have a capacity of 90 GWh — 2.5 times the size of Tesla’s largest gigafactory.
Antora Energy, a startup from California, is perhaps Rondo’s closest competitor. It is developing a thermal energy storage system that uses renewable electricity to heat blocks of solid carbon to glowing-hot temperatures in an insulated module. Founded in 2017, the company has secured $200mn in funding.
