Application of thermosyphons and insulated boards in Qinghai-Tibet DC Interconnection Project in permafrost regions

The working period of thermosyphons and the thermal effect of concrete tower footing are determined using the in-situ measured data. A coupled heat transfer model among air, thermosyphon and soil is established considering heat transfer processes within the thermosyphon without adiabatic section. Based on the model, the heat transfer processes around the tower footing and the temporal variations of thermal regime around the tower footing are studied by numerical simulation under a climate warming scenario. The results indicate that in cold seasons when thermosyphon works, the soils around the thermosyphon are obviously colder than those far away, revealing a remarkable cooling effect. Meanwhile, clod energy from the thermosyphon flows quickly to the base of tower footing due to its larger thermal conductivity. As a consequence, extensive cold permafrost develops beneath the footing. While in warm seasons, the shallow soils around the tower footing warm quickly, but those beneath the tower footing are still colder due to the cooling effect of the thermosyphon. Under the scenario of climate warming, the soils beneath the tower footing installed with four thermosyphons still keep frozen in permafrost region with the mean annual ground temperatures of -1.0℃ and -1.5℃, meeting the need of the project. But for permafrost region with the mean annual ground temperature of -0.5℃, the maximum seasonally thawing depth around the tower footing is already larger than the buried-depth of the tower footing during the later period of operation. Using the combined method of thermosyphons and insulated boards, the maximum thawing depth around the tower footing can be diminished obviously and will be less than the buried-depth of the tower footing. And meanwhile, the deep soil temperatures beneath the tower footing will also be much lower.