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