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Nant de Drance - a pumped-storage power plant in the middle of a mountain

AGCS and Allianz Suisse insure one of the most innovative power projects in the Swiss Alps. Six turbines with 150 MW each will be able to reach 900 MW capacity. This will supply about 625.000 households with electricity. Construction works started in 2008 and Nant de Drance is expected to start production in 2017.

Ever since the devastating earthquake in Japan and its catastrophic consequences for the nuclear power plant in Fukushima, it has become clear that we need to look for new solutions in terms of energy policy.

But we often forget in the debate on renewable energy that renewable energy sources, such as wind or sun, are not continuously available. It is important to be able to store energy. So how can we make electricity that is generated at peak times, i.e. when the wind is blowing or the sun is shining, available at other times as well? Electrical power grids cannot store energy, which is why there always has to be a balance between energy consumption and generation in power plants. How can we react to fluctuations in consumption or make power available quickly enough if consumption temporarily peaks?

This is a job for storage power plants. They can produce missing energy, or absorb excess energy, within only a few minutes and are reliable no matter what time of day.

How do pumped-storage power plants work?

The idea of storing energy with the help of hydroelectric power is not new: way back in the 1920s, the first pumped-storage power plants were being constructed in central Europe. The principle is straightforward: when excess energy is available, it is pumped into a reservoir situated in a higher location using hydropower. When energy demands are high, the energy can be accessed from the reservoir and channeled through turbines in a hydropower plant. The water flows downwards through turbines that power generators to generate electricity. The efficiency factor of these power plants is limited. Pumping the energy up requires more energy than the energy produced when the reservoir capacity is channeled back through the turbines. Nevertheless, these pumped-storage power plants are essential in order to compensate for fluctuations in consumption. They compensate for differences between day and night, between weekdays and weekends: the power plant pumps in the evenings/at the weekends and channels the energy back through the turbines during the day/during the week in order to generate electricity.


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Key in respect of the volume of energy to be saved is how far the water falls (i.e. the difference in altitude between the two reservoirs) and the volume of water that can be stored. In modern hydroelectric power stations, 80 to 90 percent of the pump energy invested is regained during turbining.

Pumped-storage power plants also make sense in terms of economics: at times when not a lot of electricity is being sold, the low-cost electricity is used so that, when a lot of electricity is required, electricity can be produced at a profit and sold for a better price.

Nant de Drance – a superlative pumped-storage power plant

A picturesque location in the tip of French-speaking Switzerland, overlooking Mont Blanc, is the setting for one of the most demanding power plant projects of our time.


Nant de Drance is located in the Swiss Alps with a view of Mont Blanc

With a capacity of 227 million cubic meters, the Emosson reservoir is the second-largest in Switzerland. The Vieux Emosson reservoir, which is located higher up, was developed in as early as 1955. Today, it has a capacity of 13.5 million cubic meters of water and is being lifted 20 meters for the pumped-storage power plant, increasing its capacity to 25 million. The new target location of the Vieux Emosson reservoir is 2225 meters above sea level.


The retaining wall will be expanded in the course of the project

The region is rich in history: millions of years ago, it was home to a sea, and the prolonged erosion of the glaciers created a natural strait. More than 800 dinosaur tracks were found nearby. The sea floor was lifted by the formation of the Alps and the glaciers exposed the tracks again. This tourist attraction is not impaired by the construction work, meaning that these relics of geological development remain intact for tourists to admire.

The new Nant de Drance power plant will have a pump and turbine capacity of 900 MW and will make a key contribution to securing energy supplies for Switzerland.

The aim is to commission the pumped-storage power plant on a step-by-step basis from 2017 onwards.

AGCS, in collaboration with Allianz Suisse, is providing insurance cover in the form of combined construction site insurance for the entire construction of this infrastructure project. The construction site insurance covers damage that occurs in connection with deliveries and services performed to realize the entire construction project, including all ancillary structures, and comprises construction insurance, contractor's all risk (CAR), construction principal liability, general liability and professional liability insurance.  

Since 1975, there has been a joint Swiss-French project called "Eléctricité d'Emosson" which collects some of the water from the Mont Blanc range and uses it in two hydropower plants, one on the Swiss side and one on the French side of the Emosson reservoir.
The Nant de Drance project is a partner project involving ALPIQ (54%), SBB (36%) and FMV (10%). FMV is the Valais electricity company (Forces Motrices Vallaisannes). The expansion of the project was approved with the consent of the municipalities and the population. This is one of the first new hydropower projects in Switzerland in several years.


The machinery cavern will hold the turbines for the power plant

The machine control center will be located between the two reservoirs in a cavern measuring 190 meters in length and 52 in height. The cavern is located 1700 meters above sea level. Two independent waterways, consisting of almost horizontal and vertical pressure shafts, will connect the two reservoirs. The cavern center is reached using a 5.6 km-long access tunnel starting in Chatelard. 

Current status of the Nant de Drance project

The work is currently on schedule, even though the excavation work for the access tunnel has progressed slower than planned over the last twelve months due to geological problems. Work on those construction areas located more than 2000 meters above sea level that are only accessible from the outside is suspended in winter.

The construction contract was awarded to GMI (Groupement Marti Implenia) and both the turbines and the generators were supplied by ALSTOM. The construction work includes the 5.6km long access tunnel, a machine cavern, a transformer cavern, as well as two parallel waterways, consisting of inlet and outlet structures, upstream and downstream pressure tunnels and vertical pressure shafts.

The access tunnel

How is it even possible to predict what one will find inside a mountain? A "geological forecast" can predict the layers of rock, but since this is never 100% accurate, predrilling work is also performed to clarify the imminent geological and hydrological conditions.


Nant de Drance is a huge construction site in the middle of the mountains

A tunnel boring machine with a drill head measuring 9.40 meters in diameter that eats into the mountain at a rate of six revolutions per minute can tunnel an average of 20 meters a day depending on the type of rock, although the maximum performance achieved in a day is currently 40 meters.

The aim is to have finished the access tunnel by the summer of 2012, i.e. to have reached the machine cavern. This is ambitious, because only up to half of the route through the solid rock has been managed so far. An "interference zone", i.e. a particularly challenging area of rock, had to be bored through in the process, an exceptional achievement for the whole team. The excavated rock from the tunnel is transported outside on a huge conveyor belt. Outside, it is then split into rock that can be used again, which is processed to produce concrete aggregates, and rock that cannot be used again, which is taken away for final disposal.

The access tunnel itself is a highly impressive structure, but this part is not the actual power plant that will be built here.

The caverns - the heart of the power plant

In the mining industry, the term "caverns" refers to larger artificial or natural cavities. The larger of the two caverns will be 195 meters long, 51 meters high and 32 meters wide - enough space, for example, to house a full church. It will be excavated from the top down, cast in concrete and will be big enough for the huge turbines (six vertical reversible Francis turbines, each with a capacity of 157 MW) and generators (also six vertical asynchronous motor generator units, each with a capacity of 170 MVA). Six transformers and other facilities will be located in the smaller cavern.


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During the four-month winter break, the snow will mean that, based on the current status of the construction work, it will be impossible to enter the cavern. This will change as soon as the access tunnel has been completed - then, it will be possible to reach the cavern with (relatively) dry feet. The power plant technology will not be assembled until the second project stage.

The inlet/outlet structures

The two inlet structures that are to be transported, floating, in the reservoir water to their destination are important in order to feed the water into the pressure tunnel and, as a result, into the power plant (or feed it out during the pumping process). They will be submerged in the lake shortly before the pressure tunnel will be connected to the upstream reservoir and then connected to the pressure tunnel when the water level is lower, i.e. in dry conditions. This means that they serve to let the water flow out from the upper reservoir into the lower body of water in a current-free manner once it has passed through the turbine and then let it flow back in from the lower reservoir to the machine cavern when the pumps are in operation. The concrete surface of the inlet structure has to be very smooth to steer the water flow correctly. The structure was created in dry conditions on a special platform. In order to place it in the right location on the bed of the lake, a floating assembly will be required. This procedure exploits the fact that bodies are lighter in water thanks to the lift created by the weight of the displaced water. When the Emosson reservoir is empty in the spring due to electricity production in the winter, the inlet structure can be connected to the excavated pressure tunnel.


The inlet structure was built on land and will be submerged into the water

The project manager has summarized the project as follows: "The exciting thing about Nant de Drance is that nothing is mundane. There are surprises every day, but we accept them and then work to find solutions."

So what does this uncertainty mean for the insurer? It is difficult to insure something so complex that is partially unknown. For example, it is impossible to see the reservoir bed. This means that when components are submerged to the bed of the lake, you are really going in blind. It’s essential that the insurer is involved at all times and that the construction management team and the insurer collaborate closely. This is ensured by regular inspections performed as part of a contractually agreed risk management process by Allianz Suisse and by the close contact that Allianz enjoys with Nant de Drance. A lot of work still remains to be done by all of the parties involved before this exception project can be completed. But the experience to date, the detailed and open attitude of the construction site management team - in short - the cooperative collaboration with the construction principal and the construction management team give us reason to be optimistic that the project will be successfully concluded.