Use of thermosyphons in a subarctic climate

Sammanfattning: In frozen ground, climatic changes and thermal disturbance might cause thawing and degradation of ground previously frozen. Foundations built on frozen ground might be destroyed in a matter of a few seasons. These problems are mostly pronounced in permafrost regions, but also exist in regions with more moderate climates. If thawing takes place the strength of the ground may be greatly reduced. Settlement and stability problems may therefore arise. Insulation, ventilation, or refrigeration systems are used to maintain permafrost locally around foundations. Another possible method to achieve this objective is the use of two-phase thermosyphons. By using this technique heat is transferred from the ground into the air at a higher rate than what would be the case for natural freezing only. A frozen area created in the ground will then increase in size year after year. Ski resorts in Sweden are interested in extending their ski seasons, thereby resulting in a considerable profit for them. In the autumn, the interest is mostly pronounced for cross-country skiing areas. If the ground beneath the ski track could be frozen earlier in the season, the first cover of natural or man-made snow would not easily melt away. In the spring, snow melts earlier in the ski-lift area compared to the ski slopes. If the ground could be kept frozen longer by artificial means and solar radiation is prevented, then the snow would not thaw easily and the ski-lifts could be kept open longer. The aim of this project was two-fold: to examine how well thermosyphons work in Scandinavia and other parts of the world with similar subarctic climates, and to get an insight into the design methods for thermosyphons and their installations. Installations of thermosyphons were done in the autumn of 2000 at two sites in northern Sweden. One thermosyphon was installed in Dundret, Gällivare, which is a ski resort at approximately N67° where the ground temperature as well as the climatic data has been studied. Air and ground temperatures were measured at the second installation at the university campus in Luleå, approximately N66°. This licentiate thesis consists of three parts. A technical report entitled "Ground Freezing with the use of Thermosyphons, Field Tests from an Installation at Dundret, Sweden" is devoted as "Part A", published at Luleå University of Technology, May 2002. In this report the thermosyphon installation in Dundret, Gällivare, Sweden is described. Further, ground temperature measurements for November 2000 to May 2001 and November 2001 have been analysed. In general, temperatures are lower within and closer to the installation than those in the undisturbed ground. A ground temperature difference of -6°C inside the installation area compared to the undisturbed area was observed at certain places throughout the year 2000/2001. The significant artificial cooling was noted at the beginning of February 2001. The conclusions of the study were that thermosyphons work in the climate of northern Sweden. Moreover, "Part B" is a conference paper entitled "Cooling of the Chena Hot Springs Road with thermosyphons", which was submitted and accepted to the 11th International Conference on Cold Regions Engineering in Anchorage, Alaska, May 2002. The Chena Hot Springs Road test section became operational during the autumn of 1998 and includes three test sections underlain with different types of thermosyphons plus a control section. The paper includes a description of the test site and analysis of the ground temperature measurements. It was concluded that thermosyphons could be used to cool the sub-base of the road in order to stabilise the constructions in permafrost areas. The most effective thermosyphon for the middle of the roadway was the AFI Flat unit, and the AFI Buried unit for the edge of the pavement. Lastly, a research report entitled "Ground Thermal Analysis of Thermosyphon Installations", published at Luleå University of Technology, May 2002, is devoted as "Part C" in the thesis. Heat transfer in the ground and at thermosyphon installations are explained in this report. Further, methods for dimensioning thermosyphon installations at buildings and roadways are described. A roadway and a cross-country ski track are both exemplified. In conclusion further work should be performed on developing a thermal analysis program for thermosyphon installations at roadways.