insulated panels (VIPs) are very efficient thermal barriers,
having thermal conductivity 3 to 10 times lower than conventional
issue: June 2003 APPLIANCE Magazine
Advances in VIP Design for Super Insulation of Domestic Appliances
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by Paolo Manini, Enea Rizzi, Guido Pastore, and Pierattilio Gregorio, SAES Getters
Presented at the 54th International Appliance Technical Conference (IATC), March 10-12, West Lafayette, IN, U.S.
VIPs have great potential for energy savings in a variety
of industrial and domestic applications. In spite of this,
VIPs have so far found limited use in the insulation of appliances
such as refrigerators, freezers, water heaters, or vending
machines. The main reasons for this have been the cost and
the limitation in the VIP shapes and design options available
on the market. This latter aspect is being effectively addressed
by the industry through the improvement of the manufacturing
technologies, a deep focus on component selection, and extensive
As a result, VIPs
are now available in a variety of geometries (e.g. flat, curved,
cylindrical, shaped), and with added features (holes, cut-outs),
which allow them to nicely fit in a large number of applications,
over a broad temperature range.
VIPs are produced
by vacuum packaging an open-celled, micro-porous insulating
core in a gas barrier bag. Several cores [1,2,3] have been
proposed so far, some of their properties being summarized
in Table 1.
Table 1. Some properties of currently available cores for
are available in the form of multi-layered structures, which
generally include one or more metallized sheets coupled with
high-gas barrier plastic layers. A thin metal foilÑ5
to 8 microns thickÑis added in some barrier designs
to further cut gas permeation and to improve mechanical properties.
A getter device and/or an engineered amount of desiccant is
also generally required in most VIP designs to remove gases
and moisture and to ensure long-term projected performances
Most vacuum panels
are currently being produced in the shape of flat rectangles
or squares, as shown in Figure 1, which can be easily attached
to the liner of a refrigerator or a freezer. This design cannot,
however, be applied to contour-shaped surfaces like the compressor
area of a refrigerator or to cover a round or cylindrical
surface, as is required, for example, to insulate a water
1. Example of standard flat panels.
To overcome this
problem, some novel concepts recently have been proposed.
One is based on realizing longitudinal grooves , through
machining or by mechanical compression, in the panel core.
Once the panel
has been evacuated, the applied atmospheric pressure exerts
a force on the film, and the presence of the grooves makes
the panel bend. This approach has a few drawbacks. The first
is the relatively high added cost (preparation of the grooves
is labor intensive). The second is the increased level of
stress induced on the envelope, which might be responsible
for long-term reliability problems. Another approach does
not make use of the grooves. Panels are manufactured using
the conventional technology, and then curved by means of a
special mechanical process, which ensures careful control
of the radius of curvature and surface smoothness. A typical
result is shown in Figure 2.
2. Curved panels.
These panels can
be produced using several cores in combination with a variety
of high-gas barrier envelopes, once the process parameters
are properly adjusted. This design introduces negligible stresses
in the film, as checked by specific tests carried out in SAES
GettersÕ R&D Laboratories. These tests include thermal
conductivity and pressure measurements in panels over an extended
period of time, as well as helium permeation rate measurements
through film samples taken from the panels before and after
the curving process. Due to its small molecular size and high
diffusivity in barrier films, helium is an ideal tracer to
measure changes in film properties. Its permeation rate is
proportional to the density of pin-holes or micro-defects
in the film structure. Therefore, an increase in helium permeation
rate indicates a larger level of defects in film, which in
turn, means that the barrier properties have deteriorated.
All these experiments
have consistently confirmed that the film properties of curved
panels are well within the typical required specification
values for most applications.
3. Gas inlet rate (mbar l) in flat and curved panels
versus time (@25¾C, 50 percent RH). Click
Here to see the full-size graphic.
As an example,
the pressure rate of rise as a function of time in flat and
curved VIPs is given in Figure 3. Pressure was measured over
several months by a dedicated spinning rotor gauge sensor
attached to the panels .
Results show no
substantial difference in the pressure rate of rise in the
flat and curved panels, thus confirming that film retains
full functional integrity after the panel has been curved.
Panels with Added Design Features
In a variety of
appliances including vending machines, water heaters, refrigerators,
and freezers, the presence of service pipes, feed-through
cables, and wiring poses additional requirements on VIPs.
In these cases, shaped vacuum panels with notches and holes
would be the ideal solution.
4. Examples of shaped curved panels with holes.
There are three
important issues to solve. One is related to the bag manufacturing
and sealing process. The presence of dust and particles in
the sealing area, as well as the difficulty of keeping the
sealing parameters under full control over a complex area,
have proved to be the main critical elements influencing the
quality of the final product. Another issue is how to integrate
the panel sealing process into the complete VIP manufacturing
cycle. Some cores, like silica, polyurethane foams, or glass
wool require a bake-out treatment to remove water. After being
dried-up, they can be exposed to the environment for a very
limited period of time. This aspect had to be taken into consideration
when designing the overall panel assembling process.
5. VIP long-term thermal conductivity increases for
some design options (films/getter). Click
Here to see the full-size graphic.
Last but not least,
being that cost is a critical factor for the effective adoption
of vacuum panels, the manufacturing technology must be easily
scalable to large volumes without adding substantial costs
to the finished product. All of these critical issues have
been satisfactorily solved.
In the case of
these shaped panels, extensive long-term reliability tests
have been carried out measuring pressure and thermal conductivity
over time, as described previously. Examples of shaped vacuum
panels with controlled curvature and holes are given in Figure
of Application of Curved Panels
like the ones shown in Figures 2 and 4 have been tested in
several electrical and gas-fired water heater models.
In these trials,
the panels were applied to the outer surface of the water
heater tank, partially replacing the standard polyurethane
foam or glass fiber mats. The use of these vacuum panels resulted
in much less energy consumption. Improvements in energy efficiency
as high as 25 percent have been measured . Typical results
are shown in Table 2.
2. Measured energy consumption values in a water heater
with and without vacuum panels (reference units). Results
are compared with FEA calculation.
for the replacement market, the use of VIPs in water heaters
allows for the design of efficient, ultra-slim units with
eye-catching features or the manufacture of standard-size
appliances meeting the most stringent energy requirements
without increasing the insulation volume.
Due to the relatively
high-operating temperature of the water heater (70¡C to
90¡C), the correct selection of VIP components is of the
outmost importance in this application to ensure long-term
performance and reliability. This aspect is shown in Figure
5, where different design options in terms of films and getter/desiccant
have been evaluated.
the right components are used, long-term performance can be
Curved VIPs with the design features described have also been
field-tested in a variety of other appliances with good results.
are being made to improve VIP design features. Thanks to these
efforts, vacuum panels are now available in a variety of forms
and geometries, which allow for an easy fit into a variety
of appliances, from water heaters to refrigerators.
This result has
been obtained through a careful characterization and selection
of the components and the introduction of innovative manufacturing
Evacuated Thermal Insulations: the State of the Art,Ó
Journal of Thermal Insulation, 12, 1989, H. A. Fine.
2. ÒDevelopments of Vacuum Panel Technology Based on
Open Celled Polyurethane Foam,Ó Proceeding of the Polyurethanes
Expo Õ96, 1996, W. Wacker, A. Christfreund, D. Randall,
and N.W. Keane.
3. ÒState of the Art of VIP Usage in Refrigeration Applications,Ó
International Appliance Manufacturing, 2001, B. Malone and
4. ÒThe Combogetter as a Key Component in the Vacuum
Insulated Panels (VIPs) Technology,Ó Vuoto, Apr/Jun
1997, XXVI, n¡2, P. Manini.
5. ÒMethod for Producing Non Planar Evacuated Insulation
Panels,Ó European Patent EP0820565, R. De Vos and G.L.
6. ÒThe Spinning Rotor Gauge,Ó J. Vac. Sci. Technol.
A 3(3), May/June 1985, J.K. Fremerey.
7. ÒImproved Vacuum Insulated Panel Design for Water
Heaters,Ó 5th Annual Vacuum Insulation Symposium, May
2001, P. Di Gregorio, E. Rizzi, and M. Urbano.