18 December 2015: The construction of the century awakens

It took courage, a pioneering spirit, an investment of 2.1 billion Swiss francs and ten years of planning and construction. On 18 December 2015, the time had come: the Limmern pumped storage plant supplied electricity to the Swiss transmission grid for the first time. For ten years, the project of the century has functioned like a giant battery, making it an indispensable element in the Swiss electricity supply. 

18 December 2015 is a historic moment for the Swiss energy system. At 9.57 a.m. sharp, the first of the four turbines at the Limmern pumped storage plant fed electricity into the Swiss transmission grid for the first time. This marked the start of operation of a once-in-a-century structure in the Glarus Alps, which has many spectacular features. With an installed capacity of 1000 MW, it is the most powerful pumped storage plant in Switzerland. It uses the water from Lake Limmern and Lake Muttsee, which at 2472 metres above sea level is the highest reservoir in Europe. The Muttsee dam is 1054 metres long, making it the longest dam in Switzerland. The machine hall lies 700 metres deep in the rock and is roughly the same size as the main hall of Zurich's main railway station.

Electricity for all Swiss households

The performance data is also impressive: in its ten years of operation, the Limmern pumped storage plant has produced 9.3 terawatt hours of electricity. That is about as much as all Swiss households together would consume in a year. To achieve this, the four pump turbines have turbined or pumped up a total of 6.2 billion cubic metres of water. This is roughly equivalent to the volume of Lake Thun or the annual water consumption of all Swiss households for around 14 years. Lake Mutt was emptied and refilled around 270 times. If the turbines were running at full capacity, it would take around 35 hours for Lake Mutt to be completely emptied.

Mega-battery for security of supply

However, this will not happen in reality. This is because the plant is primarily used today to compensate for  short-term fluctuations in production and demand.  "The Limmern pumped storage plant functions like a mega-battery for new renewable energies," explains Jörg Huwyler, Head of the Hydro Energy & Biomass division at Axpo (see box). This is because the strong expansion of wind energy and photovoltaics in Europe and Switzerland has led to more volatile electricity production: It fluctuates throughout the day and is dependent on the weather. When the sun is shining and the wind is blowing, there is often more electricity available than is in demand. "We use this surplus electricity to pump water into Lake Mutt". If demand exceeds production again, the water flows through the turbines back into Lake Limmern and the pumped storage plant produces electricity. "In this way, we stabilise the power grid and compensate for fluctuations caused by the weather-dependent generation of wind and solar energy," says Huwyler.

In other words: peak performance for so-called peak energy in the Glarus Alps. No wonder many people want to see this for themselves: The Limmern pumped storage plant has been offering guided tours since 2018. Since then, a total of 43,700 visitors have been shown and explained the work of the century. And: since 2025, another attraction awaits them. Axpo has set up the "Wasserwärts" photo gallery, the highest and longest photo gallery in Switzerland, in a power plant tunnel around three kilometres long.