Solar yield at 2500 metres

The Muttsee dam, inaugurated in 2016 and part of the Swiss Limmern pumped storage plant in the Glarus Alps, is the highest altitude dam in all of Europe. Another record is to add on top: A solar power plant spanning nearly one kilometre in width and with a capacity of 2.2 megawatts is being installed on the dam wall of the hydropower plant at 2500 metres above sea level. A technical and logistical challenge to generate more winter power.

Solar energy to deliver winter power – not only sounds appealing, but also offers substantial advantages: The high alpine altitude allows for significantly more direct irradiation because the sun is less often obscured by fog or cloud layers. The so-called Albedo effect, reflection of solar radiation, increases the electricity yield even more. The steep angle of the modules creates a more effective angle of incidence. In addition, the dam is already situated in a Southeast and Southwest direction. Natural cooling and ventilation will further increase module performance.

The highest solar yields are expected in the winter month of February and March. Overall the output over the year will be relatively balanced – quite the contrary to conventional rooftop installations in the Swiss Midlands. (On the potential of alpine solar plants)

Dam monitoring

The potential for solar power was identified early on during the completion of the upper reservoir stage of the Limmern pumped storage plant. In the first five years, it was necessary to closely monitor the dam before plans for a solar power plant could be advanced. (More about the Limmern pumped storage plant)

In a three-step process involving the town, the canton and the federal government, Swiss authorities granted project approval, and in January 2021, the energy supplier Axpo and IWB officially announced their intention to realise the pioneer AlpinSolar project. The Basel-based energy supplier holds a share of 49 per cent in the plant. In addition, the Swiss Federal Institute of Technology in Lausanne (EPFL) and the Institute of Snow and Avalanche Research (SLF) in Davos provides scientific support for the project in order to more closely examine alpine radiation and yield measurements.

After several years of planning, construction began in the summer of 2021.

Alpine weather is a risk factor

"Weather in the Alps is difficult to predict; there were days when work could not be done," says Christian Heierli, AlpinSolar Project Manager at Axpo.

The climate at 2500 metres is harsh and variable. Snowfall even has to be accounted for in the summer months. "At least we haven't have to perform extensive wind measurements on our own because there are two active weather stations near the dam. In combination with precipitation forecasts, this weather data was used to simulate the installation in a model and to plan for any important constraints in advance," says Heierli. The lower five metres of the wall remains free for potential snow deposits.

Two important men behind the AlpinSolar project

Bi-facial glass-glass modules

It goes without saying that the modules used must be able to withstand snow. To ensure this, 4872 bi-facial glass-glass modules manufactured by the Swiss company Megasol were installed. The 40-millimetre-wide frame fulfils the requirements for the anticipated snow load. In module areas where the highest load is expected, an additional support was installed at a central point from the back. This ensures that the glass can withstand forces and prevents sagging under pressure.

In order to use the available surface optimally, Megasol uses two different module type sizes with 460 watts and 385 watts respectively. "We assembled them onto 14.5 metre wide module tables before they were installed on the dam. We developed a completely new mounting system. That means – we created a standard module insertion system that has never been built on this scale before," says Claudius Bösiger, Managing Director of Planeco. The subsidiary of IWB is the general contractor of the project.
The primary vertical construction comprises a steel framework, but the tools for the aluminium profiles had to be especially fabricated for AlpinSolar.

Work only possible in flying weather

Snow can slide off individual modules relatively quickly owing to their steep angle of 51 degrees or, in some cases, even 65 degrees. In such quantities the snow  must not and cannot be removed manually. Higher snow accumulation than the five metres already accounted for in the simulation is unlikely.

"The construction was a daring undertaking logistically: The watch house of the Limmern hydropower plant is accessible throughout the year, but there is no road that can used for the delivery and removal of material and personnel. The entire construction site had to be supplied by transport helicopter. This was both challenging and very expensive," Claudius Bösiger points out.

For example, a lifting crane had first to be disassembled down in the valley. Then the parts were flown up to the site and reassembled on the dam wall. The pre-fabricated and wired module tables were lifted onto the wall with the crane and then fastened to the wall. As a result, construction work could only have taken place when the helicopters could fly so that all the material could be safely transported and any injured workers could be flown out with no danger. The inevitable larger CO2 footprint created by the use of a helicopter can be compensated within three months of the plant going into operation.

Dam maintenance ensured

Even after successful commissioning, AlpinSolar remains a project with special challenges: The Muttsee dam enables the solar plant to be mounted on a stable structure. However, the safety of the dam requires continuous visual safety checks. "A fissure map is prepared at least every five years. The dam must be accessible for technicians as well as in emergencies such as an earthquake. As a result, the solar modules were mounted with a 1.5-metre gap from the wall," explains Christian Heierli. 

In order to keep the solar plant accessible for maintenance work, a maintenance passage with railings was constructed in the upper section of the wall between the modules and the dam. The newly installed solar power plant was connected in a power plant tunnel 900 metres away from the technical control centre.

First PPA project in Switzerland

A long-term power purchase agreement (PPA) was concluded with Denner, the largest discount chain in Switzerland.

This is the first PPA ever for a large-scale solar plant in Switzerland. Since the Swiss nuclear power plants will be gradually taken off grid in the coming decades, CO₂-neutral power generation through tenders for solar plants on free land surfaces must be advanced (on the seven reasons for building ground-mounted systems). If Swiss politics intends to take its own climate goals seriously and achieve them, the framework conditions for the construction of large-scale solar plants must be quickly adjusted and improved.