Engadin St. Moritz Mountains Builds First thermosiphon system in Europe

After years of planning, work began last June on installing a thermosiphon system on Piz Nair as a measure against the melting permafrost. A system that has been successfully used in Alaska, Canada, and Siberia for five decades is now intended to ensure the stability and safety of the mountain station on the popular excursion destination for decades to come. All work was completed in November, and the 17 installed thermosiphons are now operational. Measurements taken next year will reveal how well this sophisticated system works in the Alps. Confidence in its success is high – as is the interest in this project, which can help adapt to the consequences of climate change in the mountains.

What happened that necessitated such a thermosiphon system at Piz Nair? – Thomas Brunner, Head of Cable Cars at Engadin St. Moritz Mountains, provided the answer in an interview with the Engadiner Post: “During the infamous terrace winter of the COVID-19 pandemic, a window that served as our service point was blocked. This single window, along with other minor cracks in the walls and ceilings, prompted us to take a closer look. We realized that the mountain station was shifting.” However, after a systematic review by a newly formed expert group of civil engineers, geologists, and surveyors, it also became clear that the safety of the cable car and the mountain station was never – and is not – at risk. Because there should be no compromises when it comes to safety, the entire mountain station was equipped with highly sensitive sensors to measure whether the ground is shifting. The investigation revealed that warmer temperatures and the associated thawing of the permafrost are the primary cause of the problems.

Permafrost only begins at depths where temperatures remain consistently below zero degrees Celsius, even in summer. When the ice melts, this "glue" is lost – and the ground loses stability. Furthermore, rainwater and meltwater penetrate to depths and rock layers previously protected by permafrost. This leads to subsidence and shifts in the subsoil during the summer and autumn months; once temperatures drop and it snows instead of raining, the ground becomes stagnant.

After thorough investigations confirmed that the operation of the cable car on Piz Nair was not in jeopardy, a solution was sought that was as sustainable as possible and designed to last for the next 30 years – and was found in the thermosiphon system. This system is designed to extract heat from the ground during the colder months, cooling it even further so that the permafrost layer, i.e., the ground beneath the mountain station, can remain frozen throughout the summer. A few degrees are sufficient, as the ground temperature only needs to be slightly below 0 degrees Celsius to maintain the permafrost. Essentially, the thermosiphons simply give nature a helping hand: in a sustainable, climate-neutral, and environmentally friendly way, since no external energy is required for the thermosiphon effect.

The principle, or rather the effect, of the thermosiphon system at Piz Nair is relatively easy to explain: Compressed carbon dioxide (CO2) circulates in each of the seventeen thermosiphons, all sealed steel pipes.

As soon as the air, and thus the condensers, falls below zero degrees Celsius, the gaseous CO2 condenses. This condensate flows down the steel pipe as droplets. Since warmer temperatures prevail there, the CO2 warms up again, becomes gaseous, and rises back up into the condenser. This creates an automatic circulation due to the temperature differences and the associated change of states of matter. The carbon dioxide cycle continuously cools the ground and thus the permafrost.

To install the thermosiphons beneath the building, a total of 580 meters of drilling was required. The drilling team faced numerous challenges: Firstly, they had to drill at unusually shallow depths, up to 45 meters into the ground, to position the thermosiphon pipes under the mountain station. Secondly, the drilling rig had to be precisely aligned to avoid damaging existing infrastructure beneath the building, such as power and water lines. Furthermore, the positions calculated by the engineers had to be adhered to with absolute precision to ensure that the influence zones of the individual pipes overlapped – a prerequisite for preserving or restoring the continuous permafrost layer. Drilling was carried out under the building from two sides. Vertical boreholes on the east and west sides complement the system, for which condensers were installed at three different locations around the station. Each thermosiphon operates independently.

In short: All good thermosiphon effects come in threes to ensure the safe operation of the Piz Nair mountain station for the coming decades.

“ This 1.8 million project is an investment in the future of Piz Nair as a tourist destination, as well as in maintaining the tourist offerings by ensuring the cable car continues to operate. We are aware that this will not solve climate problems, but rather is a situational response to a local phenomenon related to permafrost. Permafrost is always associated with mountains in our region; however, in Canada or Alaska, where this sustainable, energy-independent technology has been used for 50 years, permafrost is not found at high altitudes, but rather at sea level. And its warming, even in these colder regions, leads to ground that is no longer permanently frozen, resulting in an unstable foundation for the infrastructure built upon it. The phenomenon of thawing permafrost is therefore far more than just a mountain problem. However, the successful use of this innovative system for many years on the other side of the Atlantic gives us confidence that we can also manage the problem on Piz Nair with it.” Knowing full well that nature alone dictates and we humans must adapt. That will always remain the case .  said  Markus Meili, Managing Director, Engadin St. Moritz Mountains

Thomas Brunner, head of cable cars at Engadin St. Moritz Mountains , initiated the thermosiphon project two and a half years ago and meticulously planned its implementation together with experts – most notably Lukas Arenson, a Swiss civil engineer who has lived in Canada for 20 years and specializes in permafrost technology. The thermosiphons are now in operation, and Thomas's answers remain ready for publication.

Thomas, in a segment broadcast by SRF in mid-August 2025 (10 vor 10: "The Idea"), you sounded very optimistic. Is this feeling unchanged after the latest drilling?

Yes, I am still confident that the thermosiphons will work. We have already been able to observe and measure how heat is extracted from the ground.

Were there any unexpected challenges during the project construction, or did everything go according to plan?

We anticipated that the drilling work would be challenging, but it proved even more demanding than expected. A big thank you to the entire drilling crew, who delivered outstanding work under sometimes very adverse conditions. We completed the drilling approximately two weeks later than planned due to the challenging geology, equipment malfunctions, and weather-related interruptions. [Editor's note: The goal of completing the work by mid-October was still achieved.]

What has been the most impressive thing for you so far?

The scope and complexity of the entire Piz Nair project never ceases to amaze me. Numerous individual, sometimes highly specialized, experts are involved, spread across locations from St. Moritz to Zurich and even North America. Coordinating everything to make the best possible decisions was certainly a major challenge for everyone – even finding a meeting time that worked across all the time zones wasn't always easy… That makes me all the more impressed by the result we've worked towards for the past two years.

Did you hear any critical voices during the course of the project?

I think I was my own biggest critic beforehand! [– Thomas says with a wink] I had to delve deeply into the subject of permafrost to be convinced that this project, both in our case and overall, is the most sustainable solution. Of course, with infrastructure projects like this, there are always a few negative comments. But often, discussions can highlight the overall picture of the mountain railways and their economic importance.

You've already built cable cars all over the world. What does this project mean to you?

Even after almost 20 years in the industry, there's still something new to learn, and unconventional solutions must always be considered – whether in the Andes or right on our doorstep. Thanks to this project, I was also able to expand my personal network and meet fascinating people.

To what extent have you been confirmed so far that it was right to implement this project?

On the one hand, we were able to demonstrate that such an undertaking is indeed feasible. The project itself is generating considerable interest in the industry, extending beyond national borders, and shows that long-term solutions for maintaining infrastructure in permafrost are being sought. Furthermore, measurements taken in recent years clearly demonstrate that without intervention, continued operation of the cable car would no longer be possible in the medium term.

When can we expect "binding" measurement results that can confirm the hoped-for success of the project?

In a year at the latest, we will see whether the additional heat extraction keeps the ground frozen again throughout the summer months and thus stabilizes the building site.

What else can interested parties see of the project on site?

With the completion of construction, the platform that offered all visitors a safe and impressive view of the construction site has also disappeared. However, the finished thermosiphons and condensers remain visible. There are also plans to create an information point on the entire topic.

“Conventional anchoring with concrete anchors or even abandoning the mountain were not options for us.”

Thomas Brunner, Head of Cable Cars

Text and images: «INSIDER» magazine of Engadin St. Moritz Mountains AG https://www.mountains.ch/