Nant de Drance:
Nant de Drance pomped storage powerplant is Europe›s biggest, generating 2.5 billion kWh p.a.

The enormous underground power plant, in a cavern almost as big as the Swiss parliament building, also means enormous challenges for the contractors. Logistics are the main problem for ABAG as well, starting with crane transport through the tunnel into the powerhouse cavern last December – a technical tour de force.

This station is located in an underground cavern between the existing reservoirs Emosson and Vieux-Emosson. Its purpose is to generate power during peak demand times and smooth out the irregular production from renewable energy sources in future.
When demand is high, water from Vieux-Emosson reservoir tumbles straight down to the cavern through two penstocks 425 m long. After powering the turbines it flows into Emosson reservoir. And to store excess energy when demand is low, the turbines pump water the other way from Emosson up to Vieux-Emosson reservoir. This is how Nant de Drance pumped storage station compensates both ways for differences between supply and demand. 
The enormous underground power plant, in a cavern almost as big as the Swiss parliament building, also means enormous challenges for the contractors. Logistics are the main problem, with limited site access according to place and time. But ABAG has mastered this challenge in the Gotthard, and will do the same here starting in October 2015.