From this article it seems like they are using the sand to store heat that would otherwise go to waste due to temps too low to otherwise turn it into other form

From this article it seems like they are using the sand to store heat that would otherwise go to waste due to temps too low to otherwise turn it into other forms of energy. At least that’s what it seems like. They aren’t very specific, but it explains why they aren’t using something more akin to pumped water energy storage.