Some 120km inland in southwest Norway, more than 1,000 metres above sea level, lies the Blåsjø — Blue Sea — a three­billion-cubic-metre reservoir that sits atop a sprawling hydropower complex being looked at for development as a key “battery” in Europe’s future renewable-energy transmission network.

The reservoir, which has a capacity of eight terawatt hours (TWh) and is made up of a group of six mountain lakes fenced in by 14 dams near the village of Bykle, is the jewel in the crown of Norwegian utility Statkraft’s