FLINT, Mich. - In what he termed a counter-intuitive maneuver, Dr. Homayun Navaz, professor of mechanical engineering at Kettering University, turned down the velocity of cold air and raised the temperature to 32°F 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.
“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, they performed tests varying geometrical configurations and operating conditions resulting in approximately 3,000 data sets that were correlated through a computer program.
Navaz said, “We came up with a software based on Artificial Neural 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.”
Lower infiltration means the air is coming out at a lower velocity, said Navaz. “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,” he said.
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° and lowering the velocity of air resulted in lower suction pressure at the compressor inlet, which reduced compressor usage and therefore energy consumption.
Infiltration represents 83 percent of the cooling load and is the biggest draw on energy of refrigerated display cases. Less energy use would translate into cost savings to the tune of about $13 million for the state of California alone, according to Navaz. This energy savings would also reduce CO2 emissions. Again using California as an example, he explained it would represent a 48,783-ton reduction in CO2 emissions a year. The calculated nationwide savings for reduced infiltration rate of open vertical display cases are about $170-200 million per year with a more than 500,000-ton reduction in CO2 emissions.
Manufacturers implementing changes based on the Kettering research are already seeing better than predicted results, according to Navaz, and so far it hasn’t cost them anything to implement the recommended changes. Edison in California conducted the field tests of Navaz’s research for manufacturers and users in the state.
The Department of Energy and the California Energy Commission and Edison underwrote the research project that began in 1998 with a simple computer simulation of air curtains. The next step is researching the cold air turbulence, said Navaz. “If we can fix it at the discharge air grille we can achieve another 10 to 15 percent improvement in energy efficiency.”