As a grid-level energy storage solution, Fourth aims to compete with big lithium battery arrays in the short-duration 5-10 hour range – basically storing excess solar energy during the heat of the day for use in the evening and at night when generation drops off. But the company says it's also relevant up to the 100-hour stage, which would cover the "several days of bad weather and poor renewable generation" case.This is one of a number of thermal energy storage companies coming up out of Massachusetts and backed by Bill Gates's Breakthrough Energy Ventures fund. You might remember Antora Energy from a few months ago, with its ultra-hot carbon block batteries and high-efficiency thermophotovoltaic energy converters, for example.