Using Liquid To Cool Grocery Cases Saves Energy, Better For Environment

"There are approximately 30,000 large supermarkets in the United States, and the typical supermarket leaks about 1,500 pounds of refrigerants annually," said Predrag Hrnjak, a professor of mechanical and industrial engineering and a researcher at the university's air conditioning and refrigeration center. "This is a huge problem. In fact, supermarkets are second only to automotive air-conditioning systems in polluting the atmosphere with refrigerants."

One potential solution is to replace the high-pressure gas used in conventional refrigeration systems with a low-pressure liquid, Hrnjak said. "Circulating a liquid refrigerant ­ a technique called secondary cooling ­ can increase energy efficiency, reduce refrigerant leakage, and utilize a centralized refrigeration unit that is more reliable and less expensive to operate."

While thousands of European supermarkets have switched to the secondary cooling technology, the idea has been slow to catch on in other parts of the world. In the United States, for example, about 100 supermarkets have been converted.

One reason for the slow acceptance, Hrnjak said, is that "few tests have been performed to determine which system is actually more efficient. Nor do we know which fluid is the best one to use."

To answer such questions, Hrnjak, research engineer Yanhui Mao and graduate research assistant Wilson Terrell Jr., compared the performance of a typical supermarket display case operating with a conventional refrigerant (R404A) and with two different secondary coolants (potassium formate and potassium acetate). The researchers placed a display case, filled with simulated food packages, in an environmental chamber and monitored package temperatures as a function of refrigerant temperature.

"We found that the display case performed better with potassium formate as a secondary coolant than with the conventional refrigerant," Hrnjak said. "Our tests showed that the same product temperature could be achieved with a higher operating temperature of secondary coolant.

"For example, to maintain frozen foods at the proper temperature, the R404A refrigerant must have an inlet temperature of -32.5 degrees Centigrade," Hrnjak said. "The same product temperature, however, could be achieved with potassium formate at an inlet temperature of only -26.5 degrees Centigrade. This is a significant difference in temperature and the amount of refrigeration required."

The preliminary results "indicate that secondary cooling is a cost-effective and energy-efficient alternative to conventional refrigeration. And it's better for the environment," Hrnjak said.

The researchers presented their findings at the International Institute for Refrigeration conference, held in Cambridge, England, March 29-April 1.