The Performance of a Flat-Loop Evaporator Thermosyphon at Deadhorse, Alaska

A large (8,700 m²), single story, heated, slab-on-grade building was constructed at Deadhorse, Alaska in 2008/2009 using an array of 42 flat-loop evaporator thermosyphons to maintain the permafrost beneath the structure. The thermosyphon evaporators were installed in August of 2008 and the condensers were assembled to the evaporators in February 2009. The completed thermosyphons were activated in early March 2009. One of the thermosyphons in the array was instrumented with platinum RTD temperature sensors on the evaporator and condenser, quartz pressure transducers at the condenser end and thin film heat flux sensors on the evaporator to monitor performance. Localized air temperature and wind velocity were also measured. A data logger was installed and has been recording measurements every 3 hours continuously since the thermosyphons were activated. The data clearly shows the response of the thermosyphons at the onset of the freezing season as well as the response of the cooled subgrade during the thawing season. When operating, the instrumented 178 meter long thermosyphon evaporator is nearly isothermal with a measured temperature differential of less than 0.5°C along its length. Detailed response along the length of the thermosyphon is shown. The maximum temperature of the evaporator at the onset of the freezing season appears to have stabilized at -1.9°C after two full seasons of freezing. The methodology for sizing the thermosyphons for this structure is discussed in addition to recommended practices for designing subgrade cooling systems using flat-loop evaporator thermosyphons.