> A zero-carbon northern Europe that replaced all gas heating with heat pumps would benefit from being able to store electricity during the warm summer months, to power heating in the cold winter months.

Why store winter heat as work when you could just store it as heat or chemical energy?

All you need is a hole, some plastic sheeting, an element, and some pipes to store many GWh. Alternatively one of many cheap phase change materials, thermochemical stores, or some low grade sand unsuitable for construction.