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The Big Chill

By Dawn Hibbard

In what he termed a counter-intuitive maneuver, Dr. Homayun Navaz, professor of Mechanical Engineering at Kettering, turned down the velocity of cold air and raised the temperature to 32 degrees in a refrigerated display case in an attempt to improve energy efficiency. Ironically, it worked. Not only did energy savings go up – but the food actually got colder.

Dr. Homayun Navaz
“You would think more air coming faster would work better, but interestingly, the decreased velocity improved infiltration, which resulted in the food being one degree colder than before because the cold air was distributed more efficiently and evenly inside the display case,” explained Navaz.

“Previously, manufacturers were running the power too high on open refrigerated display cases – we just lowered the power,” he said. Using a machine called the "Proof of Concept Air Curtain" (POCAC), designed and built at Kettering by Navaz, Mazyar Amin, a Ph.D. candidate, and Professor Dana Dabiri from the University of Washington, the team performed a matrix of tests varying geometrical configurations and operating conditions resulting into approximately 3,000 data sets that were correlated through a computer program.

“Our computer model is based on experimental technology,” said Navaz, “we came up with software based on Artificial Nueral Network (ANN) that can accurately measure and minimize the infiltration of warm air into a display case. The optimal infiltration rate is a narrow bridge, but easy to hit,” he said. The software Navaz and Amin are using is based on artificial intelligence, meaning it learns through experimental or numerical information.

Lower infiltration means the air is coming out at a lower velocity. “Previously, air came out of the upper vent (or grille) of a specific display case at 90 feet per minute. We calculated the optimal speed as 65 feet per minute as an optimal discharge air velocity to yield lower infiltration rate,” Navaz said.

Lower air velocity and warmer temperature in open refrigerated display cases actually resulted in colder food and energy savings.
By reducing the velocity by 30 percent, infiltration was reduced by 12 percent and the power required was reduced by 13 percent.

Increasing the temperature at the discharge air grille by about 1 degree (F) and lowering the velocity of air resulted in lower suction pressure at the compressor inlet,which reduced the compressor usage and therefore requires less energy consumption.

Infiltration represents 83 percent of the cooling load and is the biggest draw on energy of refrigerated display cases. Less energy use translates into real cost savings to the tune of about $13 million for the state of California alone, according to Navaz.

This kind of energy savings also reduces CO2 emissions that cause green house gases. Using California as an example, he explained it would represent a 48,783-ton reduction in CO2 emissions a year. These nationwide savings for reduced infiltration rate version of just open vertical display cases can be calculated to be about $170-200 million/year with more than a 500,000-ton reduction in carbon dioxide emission.

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