Pumped water does not scale well, there are only so many mountain valleys to block. And I'd rather keep a few in a somewhat natural state (a magic "no more moun

Pumped water does not scale well, there are only so many mountain valleys to block. And I'd rather keep a few in a somewhat natural state (a magic "no more mountain valleys in exchange for instantly solved climate change" would be attractive nonetheless, even though I write this from a very nice, unblocked mountain valley). But when your concern is just seasonal storage for heating, you don't need pumped storage. Just great water in a big, isolated tank, it's not even new technology. The surface per volume shrinks the bigger the system, so very large tanks can have quite nice loss numbers.