Thermosyphons

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Thermosyphons Keep the Permafrost Frozen

Thermosyphons are self-powered refrigeration devices that are used to help keep the permafrost cool. Portions of Thompson Drive cross areas of permanently frozen ground (known as Permafrost). It is important that this permafrost layer remain frozen, but roadway construction normally produces a warming effect which can thaw it out. If thawing does take place, difficulties will arise due to the large amount of ice found in the permafrost layer. When this ice melts, it produces very wet soils and voids that are no longer capable of supporting the road. As a result, the entire roadway structure will settle and become distorted and unusable. This type of thaw settlement¯ would be difficult to repair because Thompson Drive has other improvements, such as sidewalks, curbs, and the bridge over the Alaska Railroad tracks.

Thermosyphons help to eliminate thaw settlement by refrigerating the permafrost during the winter. They accomplish this by using a refrigerant (carbon dioxide is used in the Thompson Drive thermosyphons) that boils in the lower portion of the device. As the boiling takes place, a large amount of heat is removed from the surrounding permafrost. After boiling, the refrigerant passes to the upper portion of the thermosyphon where it is recondensed. As this happens, the heat is transported out of the upper section to the cold winter-time air. This method of super-cooling¯ the permafrost during the winter allows it to survive the summer without melting.

The thermosyphons used in Thompson Drive are a new type known as Hairpin¯ thermosyphons. The photo at the top left shows the upper portion of the hairpin as construction was taking place. Now the entire device is located beneath the pavement and nothing can be seen above ground. The diagram on the lower left depicts the orientation of the hairpin thermosyphons in relation to the road surface. The red section indicates the condenser. This is where the heat is released just below the road surface so that it can make it's way to the cold winter air. The blue section is the evaporator. This is where the heat is absorbed from the permafrost layer as the refrigerant boils. In between the evaporator and condenser, a layer of insulation is installed to help keep the summer's warmth from thawing the permafrost.

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In Operation

The photo to the right shows an infrared image of the thermosyphons in operation. The bright orange stripes in the image show the warmth created at the surface of the roadway due to the release of heat from the thermosyphon condensers that are located just below the surface. The surrounding surfaces are much colder, as indicated by the blue color in the image. By increasing the surface temperature of the roadway, the thermosyphons are able to force heat to flow out into the surrounding air, thus cooling the permafrost below. Infrared imaging like that shown in the photo above is used to monitor the operation of the system.

Thanks

This project was a collaboration between the Alaska Department of Transportation and researchers at the University of Alaska Fairbanks. A total of 150 hairpin thermosyphons were manufactured for the project by Arctic Foundations, Inc. of Anchorage, Alaska.

For more about Thermosyphons in action, please contact Prof. Doug Goering.
Also, see Thompson Drive.

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