Thermosyphons are passive heat transfer devices that operate on the simple process of convection through vaporization and condensation. Heat transfer will only occur when the low end of the unit is warmer that the high end.
Ther refrigeration cycle is similar to that of mechanical refrigeration plant in that the refrigerant changes phase from liquid to vapour to liquid as it boils and condenses. The temperature differential across the thermosyphon drives the heat transfer like the compressor in a mechanical plant and the larger the temperture differential the higher the heat transfer rate.
In the context of Thermo Probes and Thermo Piles, the conductance of a thermosyphon is a function of three main resistances:
• Heat flow to the evaporator
• Thermosyphon internal resistance
• Heat flow away from the radiator
Sloping Evaporator Thermoprobes are found on the majority of existing passive subgrade cooling system installations beneath slab-on-grade structures. These units utilize a sloped evaporator to insure that the condensate will flow from the condenser to the lowest portion of the evaporator. Typically, evaporator slopes are between 10% and 3% - with the median being 5%.
Structures founded on grade over permafrost utilizing Thermoprobes for subgrade cooling also have two other very important components of design: subgrade insulation and NFS fill. the typical thermal design methodology for this type foundation economically balances the three major components. The insulation reduces the heat load on the subgrade and limits the thawing of the underlying NFS fill during the summer season when the Thermoprobes are dormant. The NFS fill contains the seasonal thaw and provides a heat sink beneath the structure. The Thermoprobes remove heat from beneath the structure primarily during the freezing season and refreeze the NFS fill so that it is completely frozen at the beginning of the thawing season.
A variation of the standard Thermo Probe, the looped evaporator does not require a constant grade to ensure the return of the condensate to all of the evaporator. By encorporating an accumulator beneath the radiator, the condensate is directed out one side of the loop and returns along with the vapour on the other side of the loop. This ensures that the evaporator walls are constantly being wetted.
The accumulator places a head on the liquid in the evaporator and as heat is picked up around the loop the ratio of liquid to gas changes within the evaporator and the density is reduced. This forces the liquid to travel in only one direction while the evaporators can be flat and undulating.
One advantage of the looped system is that installation is simplified and excavation problems are reduced. This allows longer loop lengths to be used with out requiring deep excavations. Loop lengths up to 200 m are commonly used and loops as long as 600 m have been installed. Typically the evaporators are made of 3/4" steel pipe.
Hybrid (Passive/Mechanical) Thermo Probes combine an internal heat exchanger with the normal two phase convection heat transfer of a thermo probe. A mechanical refrigeration plant can be connected to the internal heat exchanger to supplement the refrigeration. This arrangement could be used to extend the season when the ambient air temperature is too warm to probide cooling.
Hybrid units have been used to create frozen containment soils for hazardous wastes as far south as Tennesse. The hybrid unit provides total two phase heat transfer which is more efficient than sigle phase heat transfer such as a circulating cooled brine.
Hybrid units are also being used to initate freezing of dams, subgrade soils during the summer construction season and for maintaining ice surfaces in hockey and curling rinks.
Thermopiles are thermosyphons that carry a structural load. The energy-free refrigeration ability maintains lower soil temperatures resulting in increased adfreeze, shear and compressive strength of the frozen soil.
Thermopiles are used to:
• create colder temperatures allowing greater bearing capacity or increased safety factor of the pile;
• create permafrost and thus stabilize thawed or thawing soils.
• prevent froxt heaving of lightly loaded structures
The load is transferred from the pile to the permafrost soils by means of adfreeze bonding. The strength of this bond is temperature dependent and reduces to zero when the soil temperature is above 0°C. There is a very steep rise in the adfreeze strength as the soil temperature decreases below 0°C. Similarly the compressive and shear strengths of the soil also increases with a decrease in temperature.
Thermopiles reduce the soil temperatures by passively transfering heat from the soil to the atmosphere