Note: said society is preindustrial and has ample access to flowing water, but not fossil fuels or radioactive deposits. It also has access to mountain hot springs, which I thought could also be used as “batteries” during times when streams freeze. This last point is what set him off.
He also really hates wind turbines.
With the comment about the natural temperature regulation inside the mountain itself, they might be able (after massive labour) to hollow out a deep-mountain internal reservoir mostly insulated from the winter, to store potable or industrial water.