This article was originally published in the January/February 1993 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.



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Home Energy Magazine Online January/February 1993






All else being equal, getting rid of ozone-destroying chlorofluorocarbons (CFCs) will make refrigerators use more energy, while the federal appliance standards demand improved efficiency. Advanced insulations could help solve this dilemma by achieving R-20 per inch or greater, without CFCs.

A typical refrigerator-freezer fights a 60deg.F temperature differential every hour of every day--about 22,000 degree-days of annual cooling load, the heat transfer equivalent of a triple-duty Wisconsin winter--with skin insulation of only about R-15 or so. There are two ways to improve this situation: better insulation (higher R-value per inch), and more insulation (thicker cabinet walls)--both of which show promise. The push to eliminate CFCs from insulation adds to the challenge facing the industry.

Today's refrigerator cabinets are injected with 2-3 in. of CFC-11-filled polyurethane foam. The foam acts as both insulation and structural member, connecting the inner and outer cabinet walls in a quick and easy assembly-line process. Because CFC-11 is a good insulator, this foam delivers about R-7 per inch. Foams with chlorine-free blowing agents (water or pentane for example) only deliver about R-5 per inch.

Panels with nearly three times the performance of foam insulation have been built into conventional refrigerators and tested; the results show a 10-15% reduction in energy use is possible. Since even the best bulk insulation materials currently available top out at about R-8 per inch, researchers have turned to sealed panels that are either evacuated or contain low-conductivity gas such as argon or krypton. For the evacuated panels, the challenge is to devise a filler that resists the compression created by the vacuum and an outer liner that keeps air from leaking in. Both materials should also be low-conductivity.

Of several contenders in the advanced-insulation race, vacuum powder panels and vacuum fiber panels are moving the most rapidly toward incorporation into production refrigerators. Vacuum powder panels are made by taking a very fine powder (usually silica-based), encasing it in a metal or plastic liner, and sucking most of the air out of it. Vacuum fiber panels are similar, but use a fiberglass filler material. Aerogel panels are made with a slab of lightweight, translucent, silica-based aerogel, sealed in a plastic liner like a package of hot dogs.

A host of other inventions with similar performance are under development. A Lawrence Berkeley Laboratory design uses a gas fill rather than a vacuum, stacking low-emissivity plastic sheets to create panels that resemble multiple-layer superwindows without the glass. This technology still awaits testing in refrigerator units.

Installed costs for advanced insulation panels are estimated at $0.50-$2.00 per board foot, several times more than foam currently costs. U.S. Environmental Protection Agency analysis indicates that adding advanced insulation to refrigerators will increase their cost by $70-$90, yet save 125-160 kWh per year. In the coming years, this gap between foam and advanced panel costs is expected to narrow, since foams will become more expensive because of taxes on CFCs, and advanced panel insulations should become cheaper with large-scale production. Meanwhile, utility rebates could cover part or all of the cost increase.

In a project sponsored by the EPA, the Admiral Co. installed vacuum powder panels into four 19 ft3 top-mount and three 22 ft3 side-by-side refrigerator-freezers. The German firm DeGussa AG fabricated the panels, which use a precipitated silica filler encased in a plastic vacuum barrier with a nominal insulation of R-20 per inch. DeGussa specially sized the .5-.75 in. (1.3-1.9 cm) panels to fit into the test refrigerator units, covering about 63% of the cabinet surface. Unfortunately, because of the difficulty in working around multiple wire and pipe penetrations, no panels were installed on the bottom of the unit where they could have blocked heat transfer from the compressor. After securing the panels to the inside of the outer cabinet wall, the installers used conventional CFC-11-blown foam to fill the remaining void in between the cabinet walls. As a control, Admiral also assembled several conventional units on the same day, using the same assembly line and identical foam.

Admiral tested the assembled units according to a U.S. Department of Energy (DOE) procedure that measures total energy consumption in a 90deg.F ambient environment and with no door openings or food loading. The tests showed that the top-mount units with vacuum powder panels used an average of 10% less energy than the control refrigerators, while the side-by-side units used about the same. Technicians think the problem with the side-by-side units was caused by the vacuum panels bowing the doors, which in turn led to poor door sealing.

Admiral recently built and tested another series of refrigerators with advanced insulation panels, this time putting vacuum powder, vacuum fiber, and aerogel panels into separate top-mount refrigerators.

The units were built and tested in much the same way, using 19 ft3 top-mount refrigerator-freezers. In these tests, the fiber-filled panels performed the best, reducing energy consumption by 15% compared to conventional units. These panels were designed and built by Owens-Corning, using a fiberglass filler in a foil-enclosed vacuum package. The vacuum powder units repeated their previous performance of about 10% energy savings, a level also reached by the refrigerators equipped with aerogel panels.

It's too early to declare a winner among the several designs competing for this potentially large market, but during assembly of the prototypes, valuable lessons were learned that will improve the efficiency of panel sizing, panel attachment, configuration of the cabinet penetrations for wiring and piping, and foam injection. In addition, thermal modeling of the refrigerators agreed well with the measured results of both advanced and conventional units, increasing confidence in the new insulation technologies.

Another way to meet the challenge of efficient, CFC-free refrigerator production is with thicker insulation. Conventional wisdom holds that consumers want refrigerators with maximum internal volume and minimum external dimensions, meaning thin rather than thick walls. New research by DOE indicates otherwise. One hundred persons were presented with a hypothetical choice between two refrigerators, one with thick walls and one with thin walls. They were told to assume that both units have the same interior storage space, all of the features and options of importance to you (color, ice maker, and so on), would fit into the space that you currently have for a refrigerator, and would cost $500. On a 0-10 scale, with 0 indicating strongly prefer the thin-walled refrigerator and ten indicating strongly prefer the thick-walled refrigerator, the average response was 8.3, showing a definite acceptance of thicker walls. Participants also indicated that they would be willing to pay an average of $50 extra for a refrigerator if it produces less pollution and EPA certifies that it is Earth Friendly.

--David J. Houghton

David J. Houghton is a research associate with E Source in Boulder, Colo.

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