Back in 2016, Mateo Jaramillo left Tesla, where he was leading the stationary energy storage business, and started looking for a new challenge to tackle. He took on long-duration energy storage -- not long duration like 8 hours or 12 hours, but days or weeks or more. In 2017 he came on the show to talk about it.  He formed a company, now Form Energy, that has been toiling on this problem in stealth mode. Apart from saying they were building a "metal air" battery, his team held the technology close to the vest. That is, until last week.  The company announced a $200M Series D financing led by ArcelorMittal, the world's largest steelmaker, and in the process finally made public the technology, which is an iron air chemistry.  Full disclosure: Shayle led Energy Impact Partners’ investment in Form. Shayle and Mateo discuss the technology itself and the counterintuitive economics that Mateo believes will make it work. They also examine how it beat out the alternatives and how it might complement more efficient, but more expensive lithium-ion. It turns out financial modeling was far more important than spec sheets in understanding the tradeoffs. They tackle the critical question: Where exactly are the profitable applications of this technology before we hit very high 80% renewables?  They also talk about the semantics of long-duration storage vs. multi-day storage, why Mateo hates holy grails, and potential partnerships with tofu companies. The Interchange is brought to you by Hitachi ABB Power Grids. Are you building a renewable plant? Looking for a battery energy storage system? Thinking about how to integrate renewables to your grid? Hitachi ABB Power Grids is your choice.  The Interchange is brought to you by LONGi Solar, the world’s leading solar technology company. A global market leader, LONGi has unmatched bankability, quality and performance validated by third-party laboratories, and has breakthrough innovation at both the wafer and module level. See Privacy Policy at https://art19.com/privacy and California Privacy Notice at https://art19.com/privacy#do-not-sell-my-info.