Engineers at the University of Washington and Kettering University in Michigan have figured out a way to cut the energy used by the open-face coolers that are widely used in grocery stores.They say widespread implementation of their techniques across the U.S. would save roughly $100 million in electricity costs each year.
Their key finding is that a lot of cooler energy goes into overcoming the thermal load of hot air from the store aisles. So they came up with a way of orienting jets that better keep hot air out of the cooler. This can reduce energy use by as much as 15%.
Ordinary refrigerated display cases shoot jets of air across their front openings to keep cold air in and warm air out. And about 75% of their energy goes into cooling the infiltrated air. (The rest offsets heat generated by fan motors and case lighting.)
Researchers spent five years building a modular mock display case and an air curtain simulator to test various designs. They measured the amount of warm air that infiltrated for various air curtain speeds, angles, and other factors. They took the approach of trying to make minimal changes to existing cooler designs, rather than starting from scratch.
Writing in the October issue of Applied Thermal Engineering, researchers Mazyar Amin, Dana Dabiri, and Homayun K.Navaz established key variables that strongly affect the amount of warm air penetrating the air curtain. They say the most important factors are the angle between the case’s discharge and return air grilles, and the jet’s exit Reynolds number, which 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 designs.
Researchers collaborated with a display-case manufacturer to retrofit a proof-of-concept case. Navaz’s team has now established a company in Flint, Mich., that provides technical tools and training to help display-case manufacturers improve their energy efficiency.
Southern California Edison Co. funded the initial tests. Further research funding was from the U.S. Dept. of Energy, and the Calif. Energy Commission’s Public Interest Energy Research program.
The University of Washington did a news item on the research: http://www.washington.edu/news/articles/improving-the-physics-of-grocery-store-display-cases-to-save-energy
The journal article can be found here: http://www.sciencedirect.com/science/article/pii/S1359431111002997#fig1