Nant de Drance pumped storage power plant

The Nant de Drance pumped storage power plant is located 600 m below ground in a cavern between the Emosson and Vieux Emosson reservoirs in the canton of Valais. The power plant works like a gigantic battery: it is designed to store excess electricity generated by intermittent renewable energies such as wind and solar, and then to feed this energy back into the grid during demand peaks.

When electricity consumption is high, the water from the Vieux-Emosson reservoir plunges down two 425 metre high vertical shafts into the underground power station. Here it drives turbines and is subsequently fed into the Emosson reservoir. In reverse, when the demand for electricity is reduced, the water from the Emosson reservoir is pumped back up to the Vieux-Emosson reservoir. Hence, the Nant de Drance power station is able to store the energy from the excess generation. The Nant de Drance power plant has a capacity of 900 MW and has a storage capacity of 20 million kWh, corresponding to the capacity of 400,000 electric car batteries.