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This new STEP stores as much electricity as 250,000 Tesla batteries

The Nant-de-Drance WWTP in Switzerland / Image: Nant-de-Drance SA.

After 14 years of work, the Nant-de-Drance pumped storage energy transfer station (STEP) in Switzerland has just been connected to the network. A highly efficient electricity storage system to optimize the production of renewable energies.

Do you know the amazing secret hidden by a few rare hydroelectric dams? Beyond their usual-looking walls and reservoirs, some house a discreet but ingenious electricity storage system. Called “STEP” for “Pumped storage energy transfer station”, this system is a kind of battery operating exclusively with water.

The principle is simple: when there is a surplus of electricity on the network, pumps fill an upper lake. This is the recharging phase, like on a battery. Then, as needed, the water is sent to a lower lake. As it falls, it drives alternators that produce electricity. This is the discharge phase.

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Block diagram of a WWTP / EDF document published by xaintrie-passions.com

Storing large amounts of renewable electricity

Depending on its dimensions, a WWTP can thus accumulate huge quantities of electricity. A particularly useful feature in the context of the development of non-controllable renewable energies. While wind and solar only obey the weather, a WWTP can store and redistribute their production according to consumer needs.

The wind is blowing, the sky is clear, wind turbines and solar panels are producing more than necessary? The STEP activates its pumps to “recharge” the upper lake. Night falls and the wind calms down? The WWTP discharges into the lower lake producing electricity.

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Nant-de-Drance adds 20 GWh of storage and 900 MW of power

Despite this considerable advantage, very few European countries invest in new WWTPs. The commissioning of the Nant-de-Drance WWTP in Switzerland is therefore a small event. Located on the Franco-Swiss border, this unit adds 20 GWh of storage capacity and 900 MW of installed power to the Swiss and European grid. To get an idea, 20 GWh is equivalent to around 250,000 Tesla Model 3 Long Range batteries and 900 MW is the power of a first-generation French nuclear reactor.

Started on July 1, it required 14 years of work to redevelop an existing hydroelectric complex. At an altitude of 2,225 m, the Vieux-Emosson dam, which acts as the upper lake, had to be raised. A drop shaft 425 meters deep and 17 km of galleries have been drilled into the mountain. A veritable underground cathedral, the power plant houses six pump turbines of 150 MW each.

A STEP less expensive than a giga-battery

It leads to the lower lake formed by the Emosson dam, in which the water can be turbined up to a flow rate of 360 m3/s (the equivalent of the summer flow of the Rhône at the height of Geneva). The installation can pass “in less than five minutes from full power pumping to full power turbining, i.e. from -900 to +900 MW” ensures the operator, the company Nant-de-Drance. This responsiveness is greatly appreciated by electrical network managers, whose mission is to synchronize production and consumption with extreme precision. Especially since the plant has an efficiency of 80%, “one of the highest currently for storing electricity” explains the company.

The latter brings together the electricity suppliers and producers Alpiq, IWB and FMV as well as the Swiss railways SBB. The STEP required an investment of nearly 2 billion Swiss francs (equivalent to as many euros). A sum ultimately not so high in view of the amount of electricity accumulated. At €111/kWh, in the case of the Nant-de-Drance STEP, the price of storage per STEP remains lower than that of a giga-battery.

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