The Nant de Drance Hydropower Plant is a pumped-storage power station currently being constructed in the canton of Valais in Switzerland. It is within the municipality of Finhaut, district of Saint-Maurice and about southwest of Martigny. Construction on the power plant began in 2008 and it should begin operations in 2020, for a completion in mid-2021. It is owned by Nant de Drance SA, a consortium of Alpiq, SBB, Industrielle Werke Basel and Forces Motrices Valaisannes . When complete, the US$1.9 billion plant will have an installed capacity of 900 MW.
Background
On 25 August 2008, the Swiss Federal Department of Environment, Transport, Energy and Communications granted a building permit to Nate 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 and should be done by the end of 2015. The underground power house excavation was complete in April 2014. The first generators should be tested in 2017 with all six commissioned between 2018 and 2021. 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 will use two existing reservoirs to operate. The lower reservoir, Lac d'Emosson, is formed by the tall, long Émosson Dam, an arch dam which was completed in 1974. It withholds a reservoir with a storage capacity of and surface area of. The lower reservoir is long and when full, lies above sea level. The upper reservoir for the plant, Lac du Vieux Emosson, is formed by a tall and long arch dam originally completed in 1955. The dam is undergoing a raising and will be tall when done. When raised, Lac du Vieux Emosson's storage capacity will be doubled and it will withhold and have a surface area of. When full, the upper reservoir will lie above sea level. The power plant will lie in-between both reservoirs and use the pumped-storage hydroelectric method. To accomplish this, when energy demand is high, water will be released from the upper reservoir, down a series of penstocks to the six 150 MW reversible Francis turbine-generators in the power plant. The large underground power house measures long, high and wide. After generating power, water from the power plant will be discharged to the lower reservoir. When power demand is low, such as at night, the turbines will reverse and water can be pumped back up to the upper reservoir for use during high demand periods. As such, it will serve a peaking power plant. Under the guidance of AF-Consult Switzerland Ltd as general planner, the civil engineering works were carried out by GMI and the electro-mechanical works are being undertaken by GE Hydro.
