UW Today

October 13, 2011

Improving the physics of grocery store display cases to save energy

News and Information

Shoppers dont usually give a second thought as they reach into a cooler to grab milk, cheese or prepackaged lunches. Open-front refrigerated display cases, which make up roughly 60 percent of the refrigerated cases in grocery stores and supermarkets, provide quick access to chilled products such as dairy, meat, fish and produce. While they are popular with shoppers and grocery stores, theyre less popular with electric utilities  and others concerned with energy efficiency.

The experimental setup includes a half-scale display case shown in the lower left. Movable ducts adjust the settings for the air curtain that shoots across the opening.Kettering University

Engineers at the University of Washington and Kettering University are working to cut the amount of energy used by these coolers, while enhancing product safety and quality. Results published this month in the journal Applied Thermal Engineering show that tweaking the physics can reduce the energy used for refrigeration by as much as 15 percent. Lead author of the article is Mazyar Amin, a former UW doctoral student now doing postdoctoral research at Missouris Saint Louis University.

Designing grocery display cases is not rocket science, but it has a lot in common with aeronautical engineering. Refrigerated display cases shoot jets of air across their front openings, creating an invisible shield that aims to keep cold air in and warm air out.

Current technology does this with limited success.

“Most of the energy these cases use goes into cooling infiltrated air,” explains Dana Dabiri, a UW associate professor of aeronautics and astronautics. “Some energy goes to extract the heat from lighting and fan motors, some goes to remove the heat gain from radiation and conduction, but 75 percent of the cooling load is attributed to infiltration of warm and moist air from the surrounding environment.”

Open-air coolers are increasingly popular compared to other options. Refrigerated cases with doors are good at keeping cold air in, but they fog up when opened and can frustrate shoppers who want to look at more than one product while making a choice. Another design is to hang sheets of clear plastic in front of the opening, but some see this as tacky. Refrigerated bins that are open on top waste less energy because the cold air is heavier, and tends to stay inside the case. The big energy hog, and the focus of the UW research, are open-air vertical shelves.

The team includes principal investigator Homayun Navaz, a professor of mechanical engineering at Kettering who specializes in computational fluid dynamics and fluid flow simulations. Dabiri specializes in experimental work to measure and visualize fluid flows. Together they have directed five years of research in a cavernous lab on the Kettering campus in Flint, Mich.

The experimental setup includes a half-scale display case shown in the lower left. Movable ducts adjust the settings for the air curtain that shoots across the opening.

The experimental setup includes a half-scale display case shown in the lower left. Movable ducts adjust the settings for the air curtain that shoots across the opening.Mazyar Amin

“One approach is to ask, ‘What are the optimal parameters so I can get the most efficient air curtain? and then start building those,” Dabiri said. “But instead of implementing costly redesigns for existing display cases, the question became, ‘What minimal changes can I do to improve the energy efficiency of the existing units?”

The new paper establishes key variables that strongly affect the amount of warm air penetrating the air curtain. Results show that the most important factors are the angle between the cases discharge and return air grilles, and the jets exit Reynolds number, a figure that depends on the air speed and density, and the jet’s turbulence intensity.

Combining experimental results and mathematical models, the team developed a tool that lets manufacturers optimize their particular design.  Researchers collaborated with a leading display-case manufacturer to retrofit a proof-of-concept case.  Tests showed the retrofit was a cost-effective way to get a 10 percent reduction in infiltration of warm air. (Calculations for other display designs show potential savings of up to 15 percent.)

Navazs team has now established a company in Flint, Michigan, that provides technical tools and training to help display-case manufacturers improve their products energy efficiency.  “Theres definitely room for improvement in these display cases,” Dabiri said. “Weve shown that we can get 10 to 15 percent improvement, which is definitely a tangible impact. In this whole push for energy efficiency, anything you can do is a help.”

An industry-wide implementation of the findings across the U.S. would save roughly $100 million in electricity costs each year.

Southern California Edison Co. funded the initial tests. Further research funding was from the U.S. Department of Energy, and the California Energy Commissions Public Interest Energy Research program.

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For more information, contact Dabiri at 206-543-6067 or dabiri@aa.washington.edu.