The height of the Vieux-Emosson dam is raised by 20m for the Nant de Drance project. Credit: Alpiq.
The 900MW Nant de Drance pumped storage power project is being constructed between the Vieux-Emosson and the Emosson reservoirs. Credit: Alpiq.
The underground powerhouse of Nant de Drance will be accessed through a 5.6km-long tunnel. Credit: Alpiq.

The 900MW Nant de Drance pumped storage power station is being constructed in the Valais municipality of Finhaut, Switzerland.

The hydropower project is intended to generate electricity using the hydraulic forces between the Vieux-Emosson and Emosson reservoirs. It is expected to generate approximately 2.5 billion kWh of energy a year, which will be sufficient to power approximately 625,000 homes.

Nant de Drance, a partnership company, was formed for developing the Sfr1.9bn (approximately $2.1bn) pumped storage plant. The plant is owned by Alpiq (39%), Swiss Federal Railways (36%), Industrielle Werke Basel (15%) and Forces Motrices Valaisannes (10%).

The Federal Department of Environment, Transport, Energy and Communications (DETEC) Concession of Switzerland granted permission for the project construction in August 2008. Construction began in June 2009 and phased operations are expected to begin from 2018.

Nant de Drance power project details

The Nant de Drance power plant will be located between the Vieux-Emosson and Emosson reservoirs. It was initially proposed as a 600MW project comprising of four 150MW turbines. A concession modification was granted in April 2011 to increase the project capacity to 900MW.

The Vieux Emosson reservoir is situated approximately 300m above the Emosson reservoir. The height of the Vieux-Emosson dam has been increased by 20m resulting in the increase of storage capacity from 13.5 to 25 million cubic metres of water.

An underground cavern will be constructed under the mountains between the two reservoirs, as part of the project. The cavern will be accessed by a 5.6km-long tunnel from Châtelard to Vieux Emosson, which will provide year-round access to the power station. The tunnel will also act as the main conduit for the fresh air feed.

The underground cavern will comprise a power station featuring six Francis-type reversible, speed-variable pump/turbines of 150MW each. The turbines will be capable of producing energy at very short notice and enable the plant to go to full-power pumping in less than ten minutes.

The plant will also contribute to the stability of the power grid. It is expected to feed peaking power into the grid within a short time to compensate for the unpredictable power generation from renewable energy sources.

Power generation from Vieux-Emosson and Emosson reservoirs

During high electricity demand periods, water from the Vieux-Emosson reservoir (upper dam) will be released into the underground power station through 425m vertical shafts. The power station will generate electricity using the turbines in the generator mode. After generating power, water will be dispatched to the Emosson reservoir (lower dam).

When the demand for electricity is less, water from the Emosson reservoir will be pumped back to the Vieux-Emosson reservoir.

Transmission of power generated at the Nant de Drance project

Swissgrid, operator of the Swiss transmission grid network, is responsible for connecting the pumped storage project to the high-voltage grid.

For this purpose, the existing 220kV line between Châtelard and Rhone Valley will be replaced with a new 14km-long, 380kV line. The project development will also include the construction of two 380kV underground lines between Rosel and Bâtiaz, as well as two 380kV underground cables for connecting the plant’s switching system to the national grid.

Nant de Drance power plant construction

Major works at Nant de Drance include increasing the Vieux Emosson reservoir height and constructing the underground cavern that will host the powerhouse, tunnel, intake structures, hydraulic steel structures and vertical shafts.

Tunnel construction began in October 2009 and was completed in September 2012. Excavation of the machine cavern was concluded in March 2014, while the works to raise the Vieux-Emosson dam’s height were completed in September 2014.

Contractors involved

The consortium of Marti and Implenia was awarded the general construction contract for the Nant de Drance power project.

Alstom Hydro Switzerland was contracted to supply reversible pump turbines for the project. The contractual scope includes the erection, commissioning and supply of further key equipment for the plant.

ABB is the supplier of generator circuit breakers and transformers for the project. Poyry was engaged to conduct blasting and for securing the underground construction sites.

Andritz was awarded the contract to provide hydraulic steel and other auxiliary equipment required for the construction of the pumped storage power plant.

The contract for providing the electrical system for the project was awarded to Mauerhofer & Zuber Enterprises. AF Consult was engaged to conduct the project management, basic tender and final design, site supervision and commissioning works.

Doka Switzerland received the formwork contract for increasing the dam wall height and creating caverns for the project.