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Thermosyphon system creates a barrier between a building’s heat and permafrost, like “a big mattress floating on ice”

By Barb Feldman

 

About 7,000 people live in Iqaluit and fewer than 36,000 in Nunavut, one of the world’s most remote and sparsely settled regions. With more than two million square kilometres of land and water spread across three time zones, Nunavut is the largest of Canada’s provinces and territories.

No roads connect any two of Nunavut’s municipalities or hamlets, and sea access is limited to a small number of sailings between July and October. Therefore, the Iqaluit International Airport is the south’s gateway to Nunavut in the eastern Arctic, where access by air is essential for necessities like fresh food and emergency medical treatment. More than 1,500 medevac flights leave for southern cities annually, and every year 10 to 20 wide-body intercontinental airplanes land in Iqaluit for mechanical or medical emergencies, or to refuel.

Philippe Gesret was the project director for Bouygues Building Canada Inc., which along with Stantec – the Edmonton-based company provided full architectural, engineering and interior design services – was part of the Arctic Infrastructure Limited Partnership consortium
that built the new airport in partnership with the Government of Nunavut. This was the first public-private design-build-finance-operate-maintain project of its kind in North America. “It was a wonderful project – an amazing project,” said Gesret, but one that came with more than a few unique challenges.

Two new buildings – the air terminal building, about 10,000 square metres, and the central services building, just under 5,000 square metres – as well as additions and upgrades to facilities and runways, had to be constructed while the existing airport remained fully operational. These structures were built on permafrost, which consists of rock, gravel, sand and organic matter bound together by ice.

 

Permafrost thawing, instability expected to increase

Permafrost is found throughout Nunavut and is especially common in wet or coastal areas, river valleys and low-lying peaty ground. It can be as hard as rock when frozen, says Gesret, but in Iqaluit, a metre of the surface now thaws for about six weeks per year, and that depth – and the period of thawing – are both expected to increase with global warming.

The natural seasonal thawing and freezing of the surface layer commonly causes heaving and settling of slab-on-grade buildings. Most structures built in Iqaluit are set on deep pilings – not just for stability, but also to prevent snowdrifts building up at the base and to avoid melting permafrost underneath by transferring the heat of the building. Ingenious methods for keeping permafrost frozen have been employed since the 1960s, Gesret says, including refrigerated thermo-piling driven through the permafrost; the most common system in the northern territory.

“But a building has to be slightly above ground, and energy is lost through that void,” he said. Instead, for the Iqaluit Airport project, Bouygues worked with Arctic Foundations of Canada Inc., “One of very few companies in the world doing this type of work,” to design, build and install a passive thermosyphon system embedded in aggregate to form a barrier between the heat of the building and the coldness of the frozen permafrost, like “a big mattress floating or sitting on ice.”

Thermosyphons and thermo-piling use the same principle of two-phase passive cooling with high-pressure liquified carbon dioxide (CO2) as the refrigerant. As CO2 has such a low boiling point, any time the underslab temperature is warmer than the outside temperature, even to a fraction of a degree, that heat causes the CO2 to boil and turn into a vapour that rises through the piping, either into radiators arrayed outside the building, or in the case of thermo-piles, into the rings at the top of the pile. This transfers the heat out of the ground into the colder winter air, which cools and recondenses the vapour into a liquid that travels back down and continues in a loop – boiling, travelling up, condensing, going back down.

 

Passive cooling technology creates robust winter freezeback

“As soon as we activate the system, it maintains a frozen state for the life of the system,” said Justin Panagapko, one of Arctic Foundations’ owners. “It works automatically, operating faster and more efficiently as the temperature drops. So if it’s -35°C outside, the radiators allow for more condensing than when it’s only marginally colder.”

Heat transfer will only occur when the lower end of the unit is warmer than the higher end, the system isn’t designed to cycle all year round, Panagapko explains, but to create much more robust freezeback in winter to combat the increasing thaw in summer caused by rising temperatures. “With this system, there is no need for any maintenance or electrical input from the building – a passive thermosyphon installation requires zero cost per year to operate for the life of the building.”

The system, which has been engineered to work for 50 years under current conditions, includes a specially formulated one-metre deep aggregate-based pad embedded with ¾-inch carbon-steel piping and a series of sensors that monitor the temperature below the building to ensure that the permafrost remains frozen. Thermosyphon systems can also be equipped with connections to a mechanized energy-driven compression system that can cause the phase change mechanically rather than using ambient cold air, allowing for year-round freezeback if needed.

“We had very short windows of time,” said Gesret. “The system was decided upon in August 2013 and we placed the first order for materials, a kilometre of pipes, in September, which arrived on the last boat going north in 2013,” just before the sea froze. Everything in the far north starts to shut down between October and November, Gesret says.

“By doing that we were able to break ground by July and August 2014 – we had to go fast to complete 15,000 square metres of footprint in two months’ time,” including laying and treating the pipes, “before anything could be built on top of them when work resumed in the summer of 2015.” The new airport opened to passengers in December of 2017.

“The other challenge, besides logistics, is the quality of the installation – and we took it very, very seriously,” Gesret said. “If you have one nick, the CO2 will leak into the permafrost and create a pocket inside somewhere, which is going to be less or more frozen than the other part – in this case, the building isn’t floating horizontally anymore, it’s going to tilt. So you plan, you sequence, you supervise the work properly – you control, you check.”

 

“You plan, you sequence, you control, you check”

“Everything used below grade is pressure-vessel quality seamless piping,” said Panagapko, and rigorously tested for leaks using mass spectrometer helium leak detection equipment and nitrogen pressure testing once the pipes are laid.

“Anything above grade subjected to the environment is completely cleaned and sand-blasted, then coated with a flame-sprayed aluminized coating – similar to galvanizing, but much more corrosion-resistant – and then a two-part epoxy topcoat paint is applied on top of that,” he said. Although it’s important that contractors don’t dig below grade without insuring they’re aware of the system and where the piping is, he adds, modern systems are very robust, “with redundancy designed in,” so that failures on a loop, for example, will be compensated by the loop that’s next to it.

About 70 people from Bouygues were working at the site, including engineers and technicians who did the specialized work, along with workers from the local Inuit community, says Gesret. An international company with its headquarters in Paris, France, and almost 130,000 employees, Bouygues has done projects in 80 countries, but nothing like this before, he adds.

“We may not build another thermosyphon system – what matters is that we have that experience. Our first lesson was that it’s possible to build everywhere as long as you understand the local difficulties and you’re capable to work with them,” he said. “It’s mainly a question of taking the time to understand how it works and putting everything together.”

 

 

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Piling Canada is the premier national voice for the Canadian deep foundation construction industry. Each issue is dedicated to providing readers with current and informative editorial, including project updates, company profiles, technological advancements, safety news, environmental information, HR advice, pertinent legal issues and more.