United States
Environmental Protection
Agency
Air and Energy Engineering
Research Laboratory
Research Triangle Park, NC 27711
Research and Development
EPA/600/SR-93/009 March 1993
Project Summary
Advanced Insulations for
Refrigerator/Freezers: The
Potential for New Shell Designs
Incorporating Polymer Barrier
Construction
Brent Griffith and Dariush Arasteh
Current efforts to design and build
domestic refrigerator/freezers (R/Fs)
with high performance insulation tech-
nology are directed at using vacuum
panels in a composite with polymer
foam to Improve performance. However,
certain restrictions generally enable
only relatively small improvement in
thermal resistance using these tech-
niques. This report examines design
alternatives which may offer greater in-
crease in thermal performance than Is
possible with panel/foam composites.
These design alternatives involve ba-
sic redesign of the R/F and use of al-
ternative materials of construction. One
design alternative includes use of a
polymer outer shell material compo-
nent that incorporates in its construc-
tion an advanced Insulation technology
that reduces thermal bridging and edge
losses. Computer modeling of a R/F
door incorporating this concept shows
a doubling of effective thermal resis-
tance over conventional R/F designs.
The report also addresses materials and
manufacturing technologies needed to
fabricate polymer barrier advanced in-
sulation components for R/Fs.
This Project Summary was devel-
oped by EPA's Air and Energy Engi-
neering Research Laboratory, Research
Triangle Park, NC, to announce key find-
Ings of the research project that Is fully
documented In a separate report of the
same title (see Project Report ordering
Information at back).
Overview
The impending phaseout of chlorofluo-
rocarbons (CFCs) used to expand foam
insulation, combined with requirements for
increased energy efficiency, make the use
of non-CFC-based high performance in-
sulation technologies increasingly attrac-
tive. The majority of current efforts are
directed at using advanced insulations in
the form of thin, flat, low-conductivity gas-
filled or evacuated orthogonal panels,
which we referred to as Advanced Insula-
tion Panels (AlPs). AlPs can be used in
composite with blown polymer foams to
improve insulation performance in refrig-
erator/freezers (R/Fs) of conventional de-
sign and manufacture. This AlP/foann
composite approach is appealing be-
cause it appears to be a feasible, near-
term method for incorporating advanced
insulations into R/Fs without substantial
redesign or retooling. However, the re-
quirements for an adequate flow of foam
during the foam-in-place operation impose
limitations on the allowable thickness and
coverage area of AlPs. This restriction,
combined with thermal bridging effects
associated with elements such as steel
outer shells and surrounding foam, gener-
ally allows only relatively small improve-
ments in overall thermal resistance as a
result of incorporating AlP/foam compos-
ite insulation into conventional foam core
R/Fs.
This report examines design alterna-
tives which may offer a greater increase
in overall thermal resistance than is pos-
sible with the use of AlP/foam composites
in current R/F design. These design alter-
natives generally involve a basic redesign
of the R/F, taking into account the unique
requirements of advanced insulations and
the importance of minimizing thermal bridg-
ing with high thermal resistance insula-
Printed on Recycled Papei
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tions. The focus here is on R/F doors
because they are relatively simple and
independent R/F components and are
therefore good candidates for development
of alternative designs. R/F doors have sig-
nificant thermal bridging problems due to
the steel outer shell construction. A three-
dimensional finite-difference computer
modeling exercise of a R/F door geometry
was used to compare the overall levels of
thermal resistance (R-value) for various
design configurations.
One design alternative involves substi-
tuting polymer shell materials for conven-
tional steel to reduce thermal bridging and
edge losses. The computer modeling of a
simplified R/F door geometry indicated that
the percentage of improvement in overall
R-values from the use of a polymer outer
shell could be 13% for foam insulation,
15% for gas-filled AlP/foam insulation, and
18% for evacuated powder AlP/foam in-
sulation.
Another design alternative includes the
use of polymer outer shell materials but
discards foam-in-place insulation in favor
of a more comprehensive use of advanced
insulation technologies. In this case we
distinguish between AlPs and Advanced
Insulation Components (AlCs). Where an
AIP is an insulating panel made for the
inside cavity of a component, an AIC is an
entire functional component of a product
that incorporates an advanced insulation
technology. An AIC is thus a thin-walled,
hermetic, barrier part with a modified in-
ternal atmosphere and an insert consist-
ing of advanced insulation filler material.
In the case of R/Fs, an AIC could be an
entire door with accessories attached to
it. The barrier envelope, or outer surface,
of an AIC would typically be a formed (or
molded) polymer part that includes layers
of gas and moisture barrier material in a
multilayer structure. A gas-filled AIC would
have an insert consisting of a multilayer
reflective baffle and polymer stiffeners as
needed. An evacuated powder AIC would,
for example, have an insert consisting of
compressed and formed powder. AlCs
would typically not employ blown polymer
foam insulations.
The polymer barrier AIC approach of-
fers some significant advantages over us-
ing AlP/foam composite in conventional
R/F design. One of the most important
advantages is better resistance to heat
transfer resulting from greater thickness
and coverage area of the advanced insu-
lation. The polymer outer shell, in addition
to causing less thermal bridging than steel,
can offer other advantages such as de-
sign freedom, parts reduction, weight re-
duction, scrap recyclability, and process
consolidation. Polymer barrier AlCs could
be designed for disassembly giving them
an advantage in terms of post consumer
recyclability over conventional foam core
R/Fs because adhesive polyurethane
foams make it difficult to disassemble con-
ventional R/Fs.
Computer modeling of a simplified ge-
ometry (representing a 2 in. (0.05 m) thick
refrigerator door) produced overall R-val-
ues for various configurations of insulation
and shell materials as shown in Table 1.
The individual materials and manufac-
turing technologies needed to fabricate
polymer barrier AlCs are generally well
developed; however, it appears that there
have been no efforts to apply them di-
rectly to the production of AlCs. Tech-
nologies such as coextrusion and
lamination could be used to produce ther-
moplastic multilayer polymer structures
with the necessary stiffness and barrier
properties. Processes such as twin-sheet
thermoforming and coextrusion blow mold-
ing could be used to fabricate shaped
barrier parts for AlCs. Thermal and sol-
vent welding could be used to hermeti-
cally join the barrier parts.
The major conclusions of the study are
(1) AlCs could be mass produced with
existing polymer technologies,
(2) AIC R/F components can offer
higher levels of thermal resistance
than conventional assemblies in-
sulated with foam or AlP/foam com-
posites that have the same
thickness, and
(3) a considerable amount of develop-
ment is required to assess the en-
ergy efficiency improvements,
economics, manufacturing, and re-
liability of AlCs for R/F applica-
tions.
Table I. R- Values for Various Configurations of Insulation and Shell Materials
R/F door configuration
CFC blown foam w/conventional steel outer shell
evacuated AlP/foam composite w/conventional steel shell
gas filled AlP/foam composite w/conventional steel shell
evacuated AlP/foam composite w/polymer outer shell
gas-filled AlP/foam composite w/polymer outer shell
evacuated-powder polymer barrier AIC
gas-filled polymer barrier AIC
Overall Effective R-value
hr.fl2.°F/Btu(m2. K/W)
9.03 (1.59)
11.14(1.96)
9.71 (1.71)
13.09 (2.31)
11.15(1.96)
18.80 (3.31)
13.50 (2.38)
*U.S. Government Printing Office: 1993 — 750-071/60216
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B. Griffith and D. Arasteh are with Lawrence Berkeley Laboratory, Berkeley, CA
94720.
Robert V. Hendrlka is the EPA Project Officer (see below).
The complete report, entitled "Advanced Insulations for Refrigerator/Freezers: The
Potential for New Shell Designs Incorporating Polymer Barrier Construction,''
(OrderNo. PB93-146991; Cost: $17.50; subject to change) will be available only
from
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at
Air and Energy Engineering Engineering Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
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EPA
PERMIT No. G-35
EPA/600/SR-93/009
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