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…

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