The Emosson hydropower plant consists not just of the reservoir with its view of the majestic Mont Blanc massif. From January to April 2020, extensive maintenance was performed on the facilities at Les Esserts, located above the village of Le Châtelard (Swiss canton of Valais) near the French border. Through its shareholder Alpiq, Electricité d’Emosson has invested CHF 7 million in these measures, which are designed to ensure the long-term functioning of the systems and guarantee a high level of quality and safety.
The maintenance measures focused on the equalising reservoir and the pressure shaft, which allow the water flowing from the Val Ferret valleys to be stored and exploited. The dam surrounding the equalising reservoir was completely resurfaced. This was the first time the asphalt surface of the dam has been replaced since it was commissioned in 1972. The new lining ensures that the basin is impermeable and safe. The old asphalt surface was ground off and transported away to the valley for recycling. Subsequently, two new asphalt layers were applied. Eight dump trucks and 1,200 round trips between Le Trient and the Les Esserts reservoir were required to transport the old material and the new asphalt.
At the same time, the corrosion protection of the pressure shaft was renewed. After draining the shaft and stripping the old paint, a new abrasion-resistant coating was applied. Electricité d’Emosson took this opportunity to also overhaul the slide valve at the head of the pressure shaft and the outlet valve on the floor of the reservoir. These components were dismantled and transported to the valley, where they were completely overhauled in the workshops of Electricité d’Emosson and HYDRO Exploitation.
Due to the winter conditions, the construction site at 1,500 metres above sea level presented numerous logistical hurdles. The main challenges included the risk of avalanches and the constraints on applying the asphalt layers resulting from the temperature and humidity. The coronavirus pandemic created additional difficulties. In a joint effort, the employees of Electricité d’Emosson and the contractors were able to complete the work that was essential to get the facilities back into service. The measures imposed by the Swiss and French governments to prevent the spread of the coronavirus were strictly adhered to. As a result, no workers contracted the virus on the construction site. Today, the Les Esserts equalising reservoir and the pressure lines were flooded again as scheduled and can now immediately start utilising the inflowing water from the spring snowmelt. The storage power plant can thus resume operation at full capacity and supply electricity from renewable hydropower to industry and the community.
Electricité d’Emosson regularly invests in measures to maintain a high safety standard and to ensure the long-term operation of the systems. In addition, these investments ensure a high degree of operational availability of the hydropower complex with the aim of continuing to supply flexible and carbon-free electricity to help achieve a successful energy transition.
The Les Esserts reservoir was constructed between 1971 and 1972 and collects water from the Val Ferret (Saleina Glacier) and the Trient Glacier. The water flows together in La Fouly and is channelled into the equalising reservoir via a tunnel. The reservoir has a surface area of 27,000 m2 and can hold up to 220,000 m3 of water, the equivalent of approximately 60 Olympic-size swimming pools. The water collected in Les Esserts can either flow directly onto the turbines of the Vallorcine power station or be pumped into the Emosson reservoir on the other side of the valley. In summer, the Les Esserts reservoir can be used to balance the daily water inflows, which are particularly high at this time of year. The reservoir also enables water to be pumped up during periods of low electricity demand, which can subsequently be used to generate electricity when the demand is high.
Built in 1972, the pressure shaft feeds the water from the Les Esserts reservoir to the Vallorcine power station 391 metres lower down. The shaft is 508 metres long, has a diameter of 2.2 metres and a gradient of 90%. A special detail: Its head is located in Switzerland, while the valley end is located in France.
The inlet valve is located at the head of the pressure shaft. When closed, it stops the water stored in the Les Esserts reservoir from flowing down to the power station in the valley.
The Les Esserts equalising reservoir is equipped with an outlet valve, which enables it to be drained quickly in the event of impending danger. Once a year it is opened to 10 centimetres to test that it is still fully functional.