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November - December 2003

Tech Talk: Phase Change Technology

Understanding a New Concept for Insulating Glass Structures

by Terry Johnson

Phase Change Technology is a cost-effective and readily transferable method of achieving reduced solar heat gain and superior edge-to-edge R-values in multi-pane insulating glass (IG) structures. The technology takes its name from a class of temperature-activated materials that have the unique capacity to absorb remarkably large quantities of heat per unit mass (5,100 Btus/cubic foot). These include certain n-paraffins of the alkane family of organic chemicals such as n-tetradecane, n-hexadecane and n-octadecane. The materials then store that heat at their molecular bonds as latent energy and release the stored energy as heat over an indefinite number of iterations.  

What are Phase Change Materials?
The phase change materials (PCMs) of this application are organic chemicals more familiarly known as “saturated hydrocarbons,” and more specifically “straight-chain paraffins” that have an extraordinarily high characteristic latent heat of fusion. Think of a burning candle, a saturated hydrocarbon and paraffinic oil-based product. While these PCMs are by-no-means candle wax, think about how liquid candle-wax exists immediately adjacent to solid wax vis-à-vis a lit candle. The melted and solid paraffin portions of a burning candle seem to attain a point of equilibrium but in reality the candle wax continues to melt (change phase) and thereby absorb and store great amounts of energy slowly, produced by the flame, in the form of latent heat. As the candle wax pools and eventually runs over the side, it cools to its crystallization point (it changes phase from liquid to solid) and releases the stored latent heat of fusion (the extra energy of its liquid phase) to the candlestick. This rough analogy would associate the flame of the candle with the heat from the outdoors on a hot day and the candlestick with the relatively cool airspace between the glass lites of an IG unit.  

The paraffinic PCM of this product is used to mitigate heat flows through IG structures during conditions when the air-temperature on one side of the twin lite glass is significantly warmer than the air-temperature on the opposing side of the glass. The indoor/outdoor temperature gradient then moderates (outdoor temperature cools in the summer evening) sufficient for n-octadecane, the PCM between the glass panes, to freeze at 82°F. The stored latent heat of fusion is then released to the surrounding glass and the system is reset to absorb the excess heat of the next summer day.

The PCM, tetradecane, has a characteristic phase change temperature of -43°F. Consequently, this will release its stored latent heat of fusion when outdoor temperatures in the winter drop sufficient enough to cause the PCM, residing between the glass, to start to freeze despite the continuing flow of heat through the glass from the indoor heat sources. As the tetradecane freezes it must release the stored latent energy of its liquid portion to the glass very slowly. This flattens the indoor/outdoor temperature gradient between the glass panes and stops heat loss to the outdoors.

How Do PCMs Relate to IG Units? 
When these unique PCMs are injected at the IG airspace perimeter, they provide a superior resistance to interlayer heat-flow (R-value). This is achieved by combining extremely low thermal-conductivity (0.0865 Btus/hr-ft-
°F) with a capacity to exchange compensating quantities of heat with metal and glass IG substrates spontaneously. Hauser Laboratories Inc. has conducted comparative resistance to heat-flow tests (modified SIGMA TR1401-96). These demonstrate that otherwise conventional PCM charged IG units provide a 12°F improvement in minimum interior glass temperature, at the side sightline, when compared to an otherwise identically glazed IG unit fabricated with a stainless steel spacer. (R-value test air-temps: Interior 70°F and Exterior 0°F--see graphs this page).

While this technology provides superior resistance to heat-flow in indirect light or total darkness, PCMs are equally responsive to excessive heat from direct solar radiation--solar heat gain. Phase Change Technology operates by creating a discontinuity in the temperature gradient that spans the airspace and spacer adjacent to the IG perimeter.  Absent a continuous temperature gradient across the airspace, due to solar radiation or a disparity in indoor and outdoor temperatures, there can be no interior heat loss due to cold weather and no interior heat-gain from direct sunlight or 95°F in-the-shade summer heat.

A certain selected PCM (such as the ones listed at the beginning of this article) injected in a sealed stainless steel spacer, will absorb compensating quantities of excess summer heat or excess heat from solar radiation. This is sufficient enough to interrupt or flatten a rising airspace temperature gradient, as a function of the PCM’s solid-to-liquid phase transition. (Generally speaking, the resident PCM gets softer, not warmer, as the heat absorbed from glass and metal IG substrates are stored as latent energy.) At night the combination of heat inputs from interior and exterior sources cause molecules of that same selected PCM charge, now largely in its liquid phase, to decline below their liquid-to-solid phase transition temperature (-82°F). The PCM then begins to freeze and consequently release the newly stored latent energy gradually, thereby resetting the system to suppress exterior-to-interior heat-flow the following day.

Alternately, a second PCM from the same hydrocarbon group, injected into a different sealed spacer, will release compensating quantities of heat sufficient to flatten a declining airspace temperature gradient, as a function of this PCM’s liquid-to-solid phase transition (~43°F). (Freezing PCM tends to get harder, not colder, as stored latent energy is released to glass and metal IG substrates as heat.)  The graphs on page X illustrate this substantial R-value improvement. As the exterior air temperature drops to -0°F, the interior glass surface of the PCM enhanced IG test specimen retains a 12°F higher temperature than that of the conventional IG test specimen fabricated with an un-enhanced stainless steel spacer tube.         

It is important to note that when fluctuating interior/exterior temperature gradients fail to cause the resident PCM to undergo a phase transition, the system continues to be an outstanding insulator. This is due to the selected PCMs’ extremely low characteristic thermal conductivity that conducts heat at a rate more than 1,550 times lower than aluminum, 140 times lower than stainless steel and substantially lower than structural foam spacer materials.

The method, structure and materials of the PCM-enhanced IG system described above, are compatible with high-performance window and door designs and with all glass coatings, sealants and related processes and have no affect on glass clarity.  Bendable, hermetically sealed, stainless-steel spacer tubes together with desiccated matrix, primary- and secondary-edge seals are recommended. The active PCMs of this method are contained by the spacer tube, non-toxic, not a fire hazard, unrestricted by the Department of Transportation and currently available at a low unit cost. PCM supply quantities are extremely scalable. A liquid pumping and metering station and welding station, in addition to a desiccated matrix extrusion station are required within the IG fabrication plant facility. The spacer manufacturer will be required to eliminate the venting tool from his roll-form setup and to further seal the longitudinal spacer tube seam with a continuous weld. All process additions are readily adaptable to a high level of automation and process controls. An IG fabricator would incorporate pumping/metering, welding and matrix-extrusion workstations closely adjacent to a spacer tube-bending machine without further substantial change to a metallic spacer tube-oriented IG fabrication plant layout.

Our company anticipates final issue of the U.S. Patent prior to the end of year, 2003.

 

Terry Johnson serves as managing director for BTU Technologies, LLC, a technology development, technology licensing and transfer-oriented company.

 

 

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