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 ------- 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 ------- ------- 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 BULK RATE POSTAGE & FEES PAID EPA PERMIT No. G-35 EPA/600/SR-93/009 ------- |