Technologies

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Technologies

- Technologies Overview

Technologies

- Totally-Buried Thermoprobes

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This infrared image shows heat from the condensers of totally buried Thermoprobes radiating to the cold ambient air at Thompson Drive in Fairbanks, Alaska. (Photo courtesy of Doug Goering, UAF)

Thermoprobes with buried condensers are designed to release the heat picked up from the subgrade through the near surface soils or embankment. A typical application for these units is in roadway or airfield sections where heat is released just under the pavement. The condensers can be coupled to either vertical or sloping evaporators. The system is designed to provide a net cooling below the embankment section to preserve the permafrost. Because the condensers release heat through the surface, insulation can be used in the design without the icing effects that are typical of insulated sections.

In cold climates, this system is generally not designed to thaw ice and snow on the pavement, however, it will cause sublimation that will loosen the bond between the pavement and the ice. The closer the condensers are to the surface, the better this system will work.

The standard finned condenser will out-perform a buried condenser in most cases, however, the standard condensers are prone to damage and can be inundated in snow if not properly designed for the application. 

 

Technologies

- Hybrid Thermosyphons

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Hybrid flat-loop evaporator type Thermoprobes being refrigerated during building construction to accelerate construction of a service building for the new Denali Canyon Lodge located just outside the entrance to Denali National Park

Hybrid thermosyphons are designed so that they can be cooled actively as well as passively. Active cooling means that some external energy must be expended to promote cooling - mechanical refrigeration is typically used. Most any of the thermosyphons manufactured by AFI can be built as hybrid units with the addition of an internal heat exchanger.

A hybrid unit does not have to be actively cooled. The hybrid portion of the design might be to provide a higher redundancy factor over the life of the project. A typical application for hybrid thermosyphons is to accelerate construction on a project by active freezing.

Hybrid thermosyphons were used to form a frozen barrier around a decommissioned settling pond in Oak Ridge, TN in 1997 to prevent contaminants from migrating out of the area. That system ran continuously from September 1997 until April 2004 when it was shut down to allow area-wide remediation. 

AFI's hybrid thermosyphon technology compared to conventional ground freezing methods:

  • Two-phase systems are inherently more efficient than single-phase systems. The heat transfer associated with the liquid-to-gas and gas-to-liquid phase change is thermodynamically more efficient than are the heat transfer attributes of single phase liquids or gasses.
  • Two-phase working fluids, when circulated above ground, are usually liquids at temperatures which lose heat to the atmosphere instead of gaining heat, thereby improving subgrade cooling efficiency. Liquid systems circulate cold liquids which gain heat from the air, thereby wasting cooling capacity.
  • The cooling can be concentrated at predetermined subgrade depths through selective control of the wetted surface. Conventional liquid systems cannot selectively concentrate the cooling to a subgrade location because they experience their maximum temperature differentials near the refrigeration unit above ground.
  • There is no risk that accidental refrigerant leakage will reverse the freezing process. With conventional refrigeration, antifreeze liquids such as brine are not circlulated through the soils. A leak in a two-phase system would actually provide additional cooling.

Power Outages:

Power outages are of no concern in the application of hybrid thermosyphon technology. A three-dimensional finite element thermal analysis for the Oak Ridge Demonstration project predicted that it would take more than one year for the frozen barrier to toally thaw if the power were cut off. In the case of a realistic power outage (a few hours, or worst case, a few days) the wall would remain virtually unchanged. A simulated power outage of eight (8) days duration reduced the average thickness by 2.5%. 

Current state of development and certification:

For purposes of contaminant control, the most recent and pertinent application was a demonstration project at the Oak Ridge National Laboratory, in which the HRE Reactor Pond was surrounded with a frozen barrier measuring 300' long, 12' wide, and 30' deep. The system was installed during the summer of 1997, and has been operating since August of that year. The data collected continuously since startup indicates that the system is functioning as planned. It is effectively stopping the flow of radionuclide contaminants from the pond and is doing so in a very cost effective, trouble-free manner.

Technologies

- Flat-Loop Thermoprobes

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Flat-loop Thermoprobe evaporators being installed beneath a slab-on-grade shop building. The vertical white pipes are the thermosyphon risers. The condensers will be  installed on the risers after building is constructed. 
Thermoprobes with flat-loop evaporators utilize proprietary internal components within the riser to force the working fluid to travel in one direction within the evaporator. As the working fluid absorbs heat, it vaporizes and expands. The expansion helps to push the working fluid around the loop. The working fluid moves around the loop in two-phase flow, increasing in velocity as it proceeds around the loop. This allows the loop to be placed on a 'relatively' flat plane. The term 'relatively' means that there can be some undulations in the vertical profile of the loop. Typical designs allow for as much as six inches (150mm) amplitude in the undulations with no detrimental effects on the system. The subgrade can be graded to a near flat condition and the loops then placed directly on the graded material. Typically the level of the loops is approximately 1.5 feet to 3 feet (0.5m to 1.0m) below the base of the subgrade insulation. Loop lengths of 500 feet (150m) are not uncommon.

Technologies