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The Hotzenwald Group - Energy deep within the mountain

With two large underground pumped-storage power plants, Schluchseewerk AG’s Hotzenwald Group is rather exceptional. That’s because its huge pumping and power generation machines are housed deep within the mountain in artificial caves, so-called caverns, which were blasted into the hard rock in the 1960s and 1970s.

The advantage lies in the possibility of letting stored water from the upper reservoirs fall almost vertically onto the powerful turbines at the highest possible pressure without loss. The use of hydropower simply cannot be any more effective. One key feature is an additional and independent machine with black-start capability in Wehr power plant, which, with its output of around four megawatts, can be the starting point for restoring the system following a power grid collapse.

Säckingen underground hydropower plant

Säckingen underground hydropower plant is special for two reasons: it was Germany’s first underground pumped-storage power plant and it marked the foundation of Schluchseewerk AG’s Hotzenwald Group. Its four hydroelectric sets are housed in an artificial cave (cavern) within the Eggberg mountain and are reached via a 1.5-kilometre-long access tunnel.

Wehr underground hydropower plant

With a generation capacity of 910 megawatts, Wehr pumped-storage power plant is the most powerful of its kind in Germany. State-of-the-art machines move huge masses of water in a closed circuit between Wehra reservoir and Hornberg reservoir, which lies roughly 600 metres higher up. If required, power can be generated or stored within a matter of seconds – depending on power grid needs.

Technical data:

Power plant Säckingen Wehr
Upper reservoirEggberg reservoirHornberg reservoir
Lower reservoirRhine reservoirs of the Säckingen and Ryburg-Schwörstadt Rhine power plantsWehra reservoir
Drop height in metres400625
Generator output in megawatts (average):360910
Pump output in megawatts (average):296980
Max. flow rate in generator/pump mode in m³/s98/70180/144
Annual generation in MWh (rounded to the ten-year average)400,0001,100,000

Facts and figures

  • On average, the two underground power plants within the Hotzenwald Power Plant Group generate around 1,500,000 megawatt-hours of power annually – enough to supply 441,000 three-person households with electricity
  • Hornberg reservoir has a capacity of 4.4 million cubic metres of water – enough for around seven hours of maximum power generation. That is sufficient energy to bridge bottlenecks in the grid in a quick and eco-friendly manner
  • Eggberg reservoir has a capacity of 2.2 million cubic meters of water, allowing the mighty power plant turbines to be operated at full load for up to six hours
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