Two approaches garnered attention. One created a secondary loop with a familiar refrigerant used in conjunction with a glycol or brine type of product. Another approach, called distributed technology, did away with a large mechanical room and located smaller condensing units throughout the store.
At an ASHRAE seminar here titled “Analysis Methods and Field Test Results for Low-TEWI Supermarket Refrigeration Systems,” both of the methods came in for review. While both methods showed promise, neither seemed ready to immediately replace direct expansion as methodology of choice.
Presenters reported on separate research. They said lowering refrigerant charge has to be weighed against questions concerning the energy efficiency and initial costs of secondary loop and distributed technology. For the most part, they said the gap can be narrowed, but that point has not yet been reached.
Case Studies
Denis Clodic of the Ecole des Mines de Paris reported on an ongoing study of two large supermarkets in Europe. One store in France operated with a conventional direct expansion system using R-404A. The second, in Luxembourg, used a secondary loop system using screw compressors with glycol as the heat transfer fluid and ammonia.Among findings was that the temperature in the core of the meat cases of each store was lower in the direct expansion configuration. The heat exchanger surface of the secondary loop store was 70 percent larger. COP was lower in the direct expansion and energy consumption was about 20 percent lower than in the secondary loop.
Clodic said the energy penalty “can be overcome by better design” of secondary loop systems.
At the same time, he noted that many direct expansion systems are not as efficient as they should be. “Improvements are possible for direct expansion,” he said.
Ramin Faramarzi of Southern California Edison said the pluses of secondary loop includes faster defrost, lower post-defrost product temperatures, reduced leakage, reduced refrigerant costs, and lower maintenance. One the minus side, he said, is the higher energy usage and higher first costs.
He reported on two 55,000-square-foot stores in California. One had direct expansion and the other secondary loop. While the project was in its early stages, he did note that secondary systems used energy only marginally greater than direct expansion. “It’s an environmental system with a lower charge. There is a price penalty, but how can we close the gap?”
David Walker of Foster-Miller Inc. looked at both secondary loop and distributed technology. His study involved three stores in Massachusetts and New York State. One store had a conventional direct expansion system using R-404A and R-22 with air-cooled condensers. The second store used distributed technology with 10 compressor cabinets running on R-404A and placed around the perimeter of the store. The third store had a glycol loop with dry fluid coolers.
He said “direct expansion did better by 20 percent in energy consumption. Distributed used more energy because of the number of compressors in the system.”
Publication date: 04/07/2003
