Nant de Drance Hydropower Plant

The Nant de Drance Hydropower Plant is a pumped-storage power station in the canton of Valais in Switzerland. It is within the municipality of Finhaut, district of Saint-Maurice and about 14 km southwest of Martigny. Construction on the power plant began in 2008 and it began operations in 2022. It is owned by Nant de Drance SA, a consortium of Alpiq (39%), SBB (36%), Industrielle Werke Basel (15%) and Forces Motrices Valaisannes (FMV) (10%). The US$1.9 billion plant has an installed capacity of 900 MW and an energy storage capacity of 20 GWh.

Background
On 25 August 2008, the Swiss Federal Department of Environment, Transport, Energy and Communications granted a building permit to Nant de Drance SA for the construction of the plant with a 600 MW design. Preliminary construction began in September 2008 but a concession was granted in April 2011 which allowed for a larger 900 MW plant. Excavation of the tunnels began in 2009 and the caverns commenced in 2010. Raising of the upper Vieux Emosson Dam began in Spring 2013. The underground power house excavation was completed in April 2014. The first generators were tested in 2017.

In December 2014, at the International Tunnelling & Underground Space Awards, the project was awarded the Major Tunnelling Project of the Year award in the category of projects over US$500 million.

Design
The power plant uses two existing reservoirs to operate. The lower reservoir, Lac d'Emosson, is formed by the 180 m tall, 555 m long Émosson Dam, an arch dam which was completed in 1974. It withholds a reservoir with a storage capacity of 227000000 m3 and surface area of 327 ha. The lower reservoir is 4 km long and when full, lies 1930 m above sea level.

The upper reservoir for the plant, Lac du Vieux Emosson, is formed by a 45 m tall and 170 m long arch dam originally completed in 1955. The dam is 65 m tall. Lac du Vieux Emosson's storage capacity is 25000000 m3 and has a surface area of 55 ha. When full, the upper reservoir lies 2225 m above sea level.

The power plant is between both reservoirs and uses the pumped-storage hydroelectric method. To accomplish this, when energy demand is high, water is released from the upper reservoir, down a series of 425 m penstocks to the six 150 MW reversible Francis turbine-generators in the power plant. The large underground power house measures 194 m long, 52 m high and 32 m wide. After generating power, water from the power plant is discharged to the lower reservoir, releasing up to 20 GWh of energy. When power demand is low, such as at night, the turbines reverse and water can be pumped back up to the upper reservoir for use during high demand periods. As such, it serves as a peaking power plant with 80% efficiency.

Under the guidance of AF-Consult Switzerland Ltd as general planner, the civil engineering works were carried out by GMI (a joint venture of Marti and Implenia) and the electro-mechanical works were done by GE Hydro.