Advanced insulations for refrigerator/freezers : the potential for new shell designs incorporating polymer barrier construction

The impending phase-out of chlorofluorocarbons (CFCs) used to expand foam insulation, combined with requirements for increased energy efficiency, make the use of non-CFC-based high performance insulation technologies increasingly attractive. The majority of current efforts are directed at using advanced insulations in the form of thin, flat, rectangular, low-conductivity gas-filled or evacuated panels, which we refer to as advanced insulation panels (AIPs). All's can be used in conjunction with blown polymer foams to improve insulation performance in refrigerator/freezers (R/Fs) of conventional design and manufacture. This AIP/foam composite approach is appealing because it appears to be a feasible, near-term method for incorporating advanced insulations into R/Fs without substantial redesign or retooling. However, the requirements for adequate flow of foam during the foam-in-place operation limit the allowable thickness and coverage area of AIPs. This restriction, combined with the thermal bridging effects of elements such as steel outer shells and surrounding foam, allow only relatively small improvements in overall thermal resistance with AIP/foam composite insulation. This paper examines design alternatives which may offer a greater increase in overall thermal resistance than is possible with the use of AIP/foam composites in current R/F design. These design alternatives generally involve a basic redesign of the R/F, taking into account the unique requirements of advanced insulations and the importance of minimizing thermal bridging with high thermal resistance insulations. We focus on R/F doors because they are relatively simple and independent components and are therefore good candidates for development of alternative designs. We used a three-dimensional finite difference computer model of a R/F door geometry to compare the overall levels of thermal resistance (R value) for the various design configurations. One design alternative involves substituting polymer outer shell materials for conventional steel to reduce thermal bridging and edge losses. The computer modeling of a simplified R/F door indicated that this could improve overall R values 13% for foam insulation, 15% for gas-filled AIP/foam insulation, and 18% for evacuated powder AIP/foam insulation. 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 AIPs and advanced insulated components (AICs). While an AIP is an insulating panel made for the inside cavity of a component, an AIC is an entire functional component that incorporates an advanced insulation technology. An AIC is thus a thin-walled, hermetic, barrier part with a modified internal atmosphere and an insert consisting 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. It is unlikely that AICs would employ blown polymer foam insulations. The polymer barrier AIC approach offers some significant advantages over using AIP/foam composite in conventional R/F design. One of the most important advantages is better resistance to heat transfer resulting greater thickness and coverage area of the advanced insulation. The polymer outer shell, in addition to causing less thermal bridging than steel, can offer other advantages such as: design freedom, parts reduction, weight reduction, scrap recyclability, and process consolidation. Adhesive polyurethane foams make it difficult to disassemble conventional R/Fs; polymer barrier AICs could be designed for disassembly, improving post consumer recyclability. Computer modeling of a simplified geometry (representing a two inch (0.05 m) thick refrigerator door) produced overall R values for various configurations of insulation and shell materials. The results are summarized below. door configuration Overall effective R value (h ft2 °F/Btu) (m2 K/W) CFC blown foam w/conventional steel outer shell 9.03 (1.59) Evacuated AIP/foam composite w/conventional steel outer shell 11.14 (1.96) Gas filled AIP/foam composite w/conventional steel outer shell 9.71 (1.71) Evacuated AIP/foam composite w/polymer outer shell 1309 (2.31) Gas-filled AIP/foam composite w/polymer outer shell 11.15 (1.96) Evacuated-powder polymer barrier AIC 18.80 (3.31) Gas-filled polymer barrier AIC 13.50 (2.38) Full-size table Table options View in workspace Download as CSV The individual materials and manufacturing technologies needed to fabricate polymer barrier AICs are generally well developed, but it appears that there have been no efforts to apply them directly to the production of AICs. Technologies such as coextrusion and lamination could be used to produce thermoplastic multilayer polymer structures with the necessary stiffness and barrier properties. Processes such as twin-sheet thermoforming and coextrusion blow molding could be used to fabricate shaped barrier parts for AICs. Thermal or solvent welding could be used to hermetically join the barrier parts. The major conclusion of this study are: 1. (1) advanced insulated components (AICs) could be mass produced with existing polymer technologies; 2. (2) AIC R/F components can offer higher levels of thermal resistance than conventional assemblies insulated with foam or AIP/foam composites that have the same thickness; 3. (3) a considerable amount of development is required and warranted to assess the energy efficiency improvements, economics, manufacturing, and reliability of AICs for R/F applications.