Hydropower plant of Sarenne

On 1^st January 2025, CNR commissioned its new high head hydropower plant located on the torrent of Sarenne in Isère.

With an annual production of 36 GWh, it can supply renewable electricity equivalent to the electricity consumption of 16,000 inhabitants.

The story began in August 2015 with the acquisition by CN’Air (subsidiary of CNR) of the concession of the torrent of Sarenne, to build and then operate for 40 years a high head (735 m) hydropower plant with an installed capacity of 11 MW.

The Sarenne power plant: green energy production and biodiversity preservation

This structure participates fully in CNR’s strategy to develop hydroelectricity activities elsewhere than the Rhone and represents for Isère a new green energy production asset for accelerating the territory’s energy transition. It stretches over 3 municipalities in Isère:

Huez (alt.1,456 m) where the water intake is located,
La Garde-en-Oisans, crossed by the railway, completely underground along 3,660 m,
Le Bourg d’Oisans (alt.721 m) where the hydropower plant is located.

The development has been designed to maintain constant balance between economic exploitation, respect for biodiversity, and taking into account a sensitive mountain environment.

Photo caption: area where 44,000 m³ of excavated material were deposited.

Zone de dépôt des 44 000 m3 de matériaux excavés

Schéma d’ensemble des ouvrages hydroélectriques de l’Oisans montrant la prise d’eau à 1456 m d’altitude, la plateforme intermédiaire à 1000 m et la centrale au Bourg-d’Oisans à 721 m, reliées par des galeries et conduites souterraines. — Overview of the structures and their location

Illustration en coupe d’un aménagement hydroélectrique de montagne. Une prise d’eau implantée à Huez à 1456 mètres d’altitude capte l’eau et l’achemine dans une galerie souterraine traversant le massif. Le parcours passe par une plateforme intermédiaire située à La Garde-en-Oisans à 1000 mètres puis descend vers une centrale installée au Bourg-d’Oisans à 721 mètres d’altitude. Le tracé met en évidence la circulation de l’eau à travers différentes conduites enterrées et un puits incliné, sur une distance d’environ 3660 mètres et un dénivelé de 735 mètres. Des encarts en bas de l’image montrent les sections des ouvrages : conduites de 800 et 900 millimètres de diamètre placées en galerie, en tranchée ou en puits incliné, ainsi qu’une galerie en charge de 4100 millimètres de diamètre. — Cross-section representing the longitudinal profile of the water path between the water intake and the power plant

The water intake

Located at Huez at an altitude of 1,456 m, it includes:

an intake weir equipped with a fine trash rack to take the discharge for the turbine and also allow the downstream passage of fish;
a sand trap to decant abrasive materials from the torrent before it enters the penstocks;
a sand trap scour gate to ensure sediment transport;
a fish pass with 12 successive basins to ensure fish continuity and the transit of 120 l/s of compensation water.

The water conduit

The water conduit measures 3,660 m in length. It is completely underground or buried in order to preserve the mountain landscape of Oisans. It is composed of several sections between the water intake and the turbines:

a main gallery 4.20 m in diameter excavated by a tunnel boring machine that bored through the hard rock of the Oisans massif along 2.3 km on a very steep slope of 22%, then equipped with penstocks with a diameter of 900 mm.
380 m of penstocks buried under 80 cm and resting on a sand bed;

a well tilted at 45° constructed using the raise-boring technique, with a diameter of 1.8 m along 350 m, into which the penstock of 800 mm in diameter has been inserted;
an existing horizontal gallery 500 m long, dating from the beginning of the 1980s, equipped with a penstock with a diameter of 800 mm, arriving at the power plant.

The excavation spoils were treated in the perimeter of the project and deposited a few metres from the place of extraction on a deposit slope arranged for this purpose and planted with local plants.

The hydropower plant of Bourg d’Oisans

Located at Bourg d’Oisans at an altitude of 721 m, it is composed of two buildings:

the main building housing the two Pelton turbines of 5.5 MW each, a room grouping the automated controls, and a control and instrumentation room;
the switchyard enclosing the two transformers and high voltage stations.

All the structures are controlled remotely with regular visits made to all the equipment.

The works were carried out from March 2021 to September 2024, for €50 M (value in 2025).

The design and supervision of the works was entrusted to BG Ingénieurs Conseils (merged with WSP in 2024).

The industrial partners that participated in building this new development were:

Spie Batignolles GC / Cogeis / Edilmac: for the underground infrastructures;
Idroweld: for the penstocks;
Eiffage GC: for the civil engineering of the water intake and the hydropower plant;
SDEL Actemium: for the hydro-electromechanical equipment (Turbines: ATB Riva).

Sarenne hydroelectric power station Durée : 3:45 min

Inauguration of the Sarenne hydroelectric power station Durée : 1:57 min