The hydroelectric power plant on Lake Limmern, which was built in the 1960s and is majority-owned by Axpo and minority-owned by the canton of Glarus, was expanded in 2014 to include Lake Muttsee. At 2474m above sea level (at full supply level), this is not only the highest reservoir in Europe with the longest dam wall in Switzerland, but also the first major new reservoir in Switzerland since the 1970s. A pumped storage plant with a total of 1000MW installed pumping and turbine capacity was built between the two lakes, the largest in Switzerland. The entire hydropower plant has further pumping and, above all, turbine capacities for several expansion stages.

Even before the Muttsee Dam was completed, the idea of installing a photovoltaic system on it materialised. It was expressed, for example, in letters to the editor in the regional newspaper, so it was present in many people's minds. The gravity dam, which faces perfectly south-southeast to south-southwest, with a downstream slope of 51 degrees at the bottom and 90 degrees at the crest, was an invitation to do so. Due to the more intensive dam monitoring in the first five years of operation, it was agreed that a PV system from the hydropower plant point of view would be possible, but not until 2020 at the earliest. 

Unprofitable yet attractive 

When the green light to start the project came in 2017, the return on investment had to be considered insufficient and thus the realisation questionable: the Swiss solar subsidy was geared towards self-consumption and was low for the project (not even 10% of the investment), with construction costs higher than on the Swiss Plateau. 

However, there were concrete reasons why we still wanted the project.  At almost 2500m asl, the productivity of solar plants per unit area is about 50% higher than on the Swiss Plateau. PV systems on the Swiss Plateau only generate a quarter of their output in winter whereas about half of the potential Muttsee production occurs in the winter half-year – when electricity demand is high. The reasons for this are the numerous hours of sunshine in the fog-free mountains in winter, the additional yields due to the reflection of light from the snow present here, and the lower temperatures as panel efficiency is temperature-dependent. It was thus foreseeable that such alpine systems could become very attractive in the longer term – as soon as the assumed electricity shortage in winter would be reflected in electricity prices and as soon as the construction of large PV systems in the Alps would become legally possible.

A changed environment

Much faster than anybody thought, the environment for alpine solar installations is changing. Prices are high and the Swiss parliament seems to be ready to relax the conditions for installing panels in the Alps. PV on infrastructure (such as in the Muttsee case) should be supplemented with PV near existing grid and road infrastructure (near hydropower plants, ski areas and the like). 

The lack of profitability of the specific project, but good prospects for follow-up projects and the desire to gain experience had already led Axpo to cooperate with IWB, the utility of the city of Basel. It was Planeco, IWB's PV subsidiary, which built the plant together with the subcontractors. The two investors were joined by the Swiss discounter Denner, which secured the entire production over 20 years in a power purchase agreement.

Once the cooperation between IWB and Axpo, the supply contract with Denner and the work contract with Planeco were in place, and Axpo and IWB had approved the investment, we could begin. 

Due to the extreme weather conditions and the large amounts of snow, construction can only be carried out in summer and autumn until the snow has fallen. The spring of 2021 was very snowy and then the weather afterwards often rainy. 

So, what exactly is it that makes working in the high mountains really demanding? The main answer is the snow that interrupts construction activity in winter. It can cover the panels (hence the steep arrangement and the free space at the base of the wall) and cause frost damage by thawing and freezing of snow and ice. A second point is access, which was only possible here by helicopter. 

However, that was not the reason for the delays in construction. These were caused by pandemic-related delivery difficulties in obtaining materials, such as the primary substructure of the plant, which fixes the panels at a distance of 1.5 m in front of the wall. As a result, the site had to be winterised towards the end of 2021 and the work completed in the summer of 2022. 

Future projects

Hydropower is the most important pillar of Swiss electricity generation, with photovoltaics likely to become number two in the longer term. While, from the energy industry's point of view, increased storage facilities for hydropower in Switzerland are particularly valuable (e.g. projects to raise dams), the general production issue in Switzerland is winter electricity: in summer there are surpluses, in winter there is a need to import (but the capacity ability to import is limited), and this imbalance is likely to worsen as more nuclear power plants are shut down and more conventional PV (with a production share in the winter season of a good 25%) is added. From this point of view, alpine PV plants are very desirable – even more so as wind power (with its high winter share) has had an extremely difficult time in Switzerland so far.


Muttsee AlpinSolar was borderline from a profitability point of view, and the potential for alpine plants that could actually be built was initially very low. With current electricity prices, profitability looks very different, and the planned changes in the law should make PV plants possible at many alpine locations. The location at a dam is likely to be the exception, but hydropower plants have considerable potential for new alpine PV plants due to their existing infrastructure ( ie electricity and road connections in the high alpine region).