Energy storage: power from the Alps

Allianz Corporate & Specialty (AGCS) and Allianz Suisse are insuring one of the most innovative new hydroelectric power plants in Switzerland.
View from the higher Vieux Emosson reservoir to the Lac d\'Emosson, the second-biggest reservoir in .../ Credits: Michel Martinez
It’s the age-old dilemma: you can produce environmentally-friendly electricity but you can’t keep any in reserve. At night and on windless days even the best solar and wind power stations are useless.

Electricity grids can’t store energy, so energy consumption and energy production in a power plant must always be balanced. But how can suppliers respond to consumption fluctuations or short-term peaks quickly enough to produce the available capacity?

Pumped-storage hydroelectric power plants are one answer. They can produce the energy needed within minutes and store surplus energy reliably, day or night.

The principle is simple: when surplus energy is generated, it is used to pump water from a lower reservoir to one further uphill, which can be used to generate power during peak consumption periods. The water is channelled downhill through turbines, which in turn drive generators.

The concept of storing energy with the aid of water power isn’t new: the first pumped-storage power plants were built in Central Europe in the 1920s. However, the efficacy of such a power plant is limited. Pumping the water uphill requires more energy than can be recovered.

Nevertheless, these pumped-storage plants are essential to compensate for fluctuations in demand. Cheaper electricity produced in off-peak periods, in the night for instance, is used to pump water to the upper reservoir, which is then used to produce profitable electricity when demand is high.

A view of Mont Blanc

One of the most modern pumped hydro facilities in the world is currently being built in the Swiss Alps in a picturesque corner of French-speaking Switzerland with a view of Mont Blanc.

Six 150-megawatt turbines, powered by stored energy in the form of pumped water, will drive the Nant de Drance Alpine power station, supplying 625,000 households with electricity.

It will connect two reservoirs: the Lac d’Emosson, which at 227 million cubic meters is the second-biggest reservoir in the country, and the higher Lac du Vieux Emosson reservoir, 2,200 meters above sea level which currently holds 13.5 million cubic meters of water. The dam wall will now be raised by 20 meters, which will almost double the capacity.

The turbine station will be built in a 190-meter-long, 52-meter-high cavern situated between the two reservoirs at 1,700 meters above sea level. Two separate water channels will connect the two reservoirs.

The construction work includes a 5.6km-long access tunnel, a machine cavern, a transformer cavern and two parallel waterways consisting of inlet and outlet structures, upstream and downstream pressure tunnels and vertical pressure shafts.

The water will be fed into the pressure tunnel and from there to the power plant via two inlet structures, which will be floated along the reservoir to their destination. They will be submerged in the reservoir shortly before the pressure tunnel is linked to the upstream reservoir and will be connected to the pressure tunnel when the water level is low. The structure was built on a special platform in dry conditions and then placed on the reservoir bed.

The larger of the two caverns will be 195 meters long, 51 meters high and 32 meters wide – large enough to contain a good-sized church. It will be excavated from the top down and lined with concrete. When completed, it will house six huge turbines and six generators. The smaller cavern will contain six transformers and other equipment.

The new plant, with its 900-megawatt pump and turbine capacity, will contribute significantly to Switzerland’s future energy security. Begun in 2008, it will come on stream in stages from 2017.

Together with Allianz Suisse, Allianz Global Corporate & Specialty (AGCS) is insuring the entire construction of the infrastructure project. Most of the facility will be underground. According to the operating company, the power plant will have the minimum impact on nature and the environment.

Surprises every day

The construction work is on schedule, even though the excavation work for the access tunnel has taken longer than planned due to geological problems. In Winter, work stops on all construction sections above 2,000 meters, which can only be reached from the outside anyway.

To avoid any unpleasant surprises in the mountains, test bores were sunk at several sites before construction began to investigate the geological and hydrological conditions. A tunnel-boring machine with a 10-meter drill head eats into the mountain at an average rate of 20 meters a day, depending on the type of rock. The record for one day currently stands at 40 meters.

“The most exciting thing about Nant de Drance is that nothing is run of the mill,” says project leader Traugott Benz. “There are surprises every day but we take things as they come and find solutions.” However, such surprises can be problematic for an insurer.

“It’s therefore crucial that we’re kept up-to-date about what’s happening on the construction site and that there’s close cooperation between the project leader and the insurer,” says Rolf Vetsch, who is responsible for power plants and alternative energy projects at Allianz Suisse.

Vetsch, a mechanical engineer himself, can be satisfied with progress so far: to date on this billion-dollar project no major loss has occurred.

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