Clean energy storage is a billion-dollar problem — but Elon Musk aside, a number of technologists are eyeing existing infrastructure and wondering how it could, effectively, serve the function of a giant battery. Take the California utility proposal Next City reported on earlier this year. Because school buses operate on fixed schedules and sit idle during peak demand times, Pacific Gas and Electric Co. wants to study whether electric school buses can effectively store power from renewables, and then send that power back to the grid when necessary.