Thermosyphons and Their Use in Engineering

Constructing roads, buildings, dams, rail lines, and hockey rinks in cold climates where the ground is permanently frozen poses considerable challenges. When a structure is placed on top of permafrost ground, the ground can melt and cause it to heave. This often results in structural damage to the building on top.

It does not help to place insulation material on permafrost soil. All that does is starve the soil from natural atmospheric freezing leading to more melting. So, how can structures maintain stability in frigid climates with as little damage to the environment as possible?

Engineers have solved the problem with thermosyphons. These tubular structures are inserted deep into the ground to provide a solid foundation for construction projects. Thermosyphon tubes can be up to eight inches in diameter and can be filled with liquid carbon dioxide (or other gasses) under pressure.

Inside a thermosyphon tube, gravity pulls the liquid down to colder and colder temperatures below. When that liquid hits a specific temperature and pressure combination, it “boils” and begins to evaporate. This causes carbon vapor to rise upward through the tube. It draws heat away from the soil as it does so, letting the soil remain frozen.

The flow that occurs inside a thermosyphon tube is a passive system because it requires no outside energy to drive the process. This action is called convection. The differential in ground temperatures from layer to layer is what drives the system.

Carbon dioxide is not the only substance that can be used. Thermosyphons can be loaded with helium, hydrogen, argon, and neon. It should be noted that while thermosyphons work naturally without an energy driver, some designs employ a cryocooler to deliver even more cooling at the cold reservoir end.

Because they are driven by the natural force of gravity, thermosyphons are limited to use in vertical or near-vertical situations. Also, the distance between the top and bottom of these systems must be sufficiently large to drive the process. Another key factor is the design. A thermosyphon loop must be constructed in such a way as to avoid pockets that can trap the flow of returning warmer air.

Today, thermosyphons see wide use in Alaska, Canada, and Russia. In these locations, extremely cold conditions have required innovative solutions to provide stable foundations for all kinds of buildings, rail lines, and other items situated atop permafrost.

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