Vieux Emosson Reservoir

Stabilizing the European grid and securing Switzerland's power supply

900 MW Nant de Drance Pumped Storage Power Plant in the Swiss Alps

The Nant de Drance pumped storage facility was designed to exploit part of the enormous hydroelectric potential of the Swiss Alps for stabilizing the European grid and securing Switzerland's power supply. It is one of the most powerful pumped storage plants in Europe and has a storage capacity of 20 million kWh.

AFRY was commissioned by the client in 2005 to develop this project and to bring it to implementation maturity via the feasibility study, the preliminary, construction and conditional project as well as via the tenders. Construction work began in autumn 2008, with AFRY in various key roles, such as technical coordination, implementation planning and construction supervision.

Our expert services included technical planner for construction, electromechanics, electrical engineering and control technology, and providing services in construction and installation management as well as in commissioning.

Commissioning is scheduled for the end of 2021.

View of Emosson Reservoir from Vieux Emosson
Emosson Reservoir

Power plant features

The 900 MW Nant de Drance pumped storage power plant was developed incorporating the two existing reservoirs Vieux Emosson (upper reservoir) and Emosson (lower reservoir) in the Valaisian Alps along the Swiss and French border. The available head between these two reservoirs is up to 400 m. The capacity of the upper reservoir was doubled by increasing the height of the existing dam wall by 20 m.

The headrace system is designed in two parts. Each part consists of approx. 1500 m of pressure tunnels and a 440 m deep vertical shaft. Almost 15 km of tunnels provide access to the facilities.

The underground power plant complex mainly consists of the main cavern (195x30x52 m), the transformer cavern and three access tunnels. The longest tunnel is the main access tunnel to the machine cavern.

It is 5.6 km long, driven by a hard rock TBM with a diameter of 9.45 m and a maximum gradient of 12% (winter-proof access). Six reversible pump-turbine units with 150 MW pump/generator capacity each are installed in the power plant cavern.

Powerhouse cavern with electrical equipment
Electrical component installation in the powerhouse

Nant de Drance as a project of the century

With costs of just over 2 billion Swiss francs, the project will be an important component of the Swiss and European energy strategy. Thanks to the installed capacity of 900 MW, the power plant will be able to produce large amounts of electricity or store energy within a very short time. The high flexibility will be one of the main strengths of this plant, as the volatile amount of electricity generated from the new, renewable energy sources can be optimally stored in this way.

Martin Aemmer
Head of Business Unit Hydro, Switzerland, Energy Division

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