Even though direct expansion systems dominate the supermarket industry, a report from Hill-Phoenix noted recent developments with secondary-coolant technology and general industry trends.

Driving factors, according to the report, are "severe competition, small profit margin, high energy cost, high refrigerant price, regulatory pressures, and public perception/image."

The report said, "The secondary-coolant technology has evolved in the last decade as the most reliable solution to these factors mainly through refrigerant charge reduction, refrigerant leak elimination, maintenance simplification, and product quality improvement. New advanced designs and operational features are applied for energy parity with the traditional centralized direct-expansion system. These features include lower floating condensing pressure, deeper liquid subcooling, lower vapor superheat and pressure drop in the refrigerant return lines, simpler oil management, and reduced or eliminated oil accumulation in the heat exchangers/coils."

The report went on to say, "Additional benefits of the secondary coolant systems are improved product quality and reduced shrink in fresh foods, the opportunities to use more efficient and environmentally friendly refrigerants, and to reduce the demand and dependence on qualified technicians during installation and operation."

A team from Ingersoll-Rand Climate Control USA did an energy analysis of various supermarket refrigeration systems with regards to TEWI (Total Equivalent Warming Impact) and annual operating cost.

According to the report, "The systems which were modeled in detail include parallel racks, distributed, self-contained, glycol secondary loop, and CO2 secondary loop (medium temperature) and cascade (low temperature). Based on R-404A modeling results, distributed systems with scroll compressors have energy usage 6 to 9 percent lower than the baseline parallel rack system. On the other hand, self-contained units with horizontal scroll compressors and water-cooled condensers have energy consumption 11 percent higher than parallel racks, and glycol fluid secondary loop systems have energy consumption 15 percent higher than parallel racks. CO2 secondary loop/cascade systems with propane as primary refrigerant have energy consumption comparable to parallel racks. The CO2 systems also have low TEWI."

A team of engineers from Aalborg University, Technical University of Denmark, and Danfoss, all based in Denmark, looked beyond what they called the traditional control method of commercial refrigeration systems, which focus on controlling the air temperature inside the refrigerated display cabinet.

Their paper discussed what was called a "dynamic heat transfer model." They developed what they called a "one-dimensional dynamic model to which an Explicit Finite Difference Method is applied to handle the unsteady heat transfer problem with phase change, as well as time varying boundary condition."

They said, "This model can serve as a prerequisite for modeling of food quality changes, thus enabling the possibility of improving the control of a supermarket's refrigeration system. This model uses simple explicit FDM to deal with the phase change problem, featured with time varying boundary condition, and temperature independent thermal physical properties. It can be used as one tool to evaluate the effect of various parameters to the product temperature change."

The research concluded that "the defrost cycle in the refrigeration system influences the product temperature dramatically. For some type of fish, low water content can lead to higher temperature change, while for different type of fish we need to consider the overall effect of diffusivity. By simulating the different defrosting scenarios, we can obtain the different product temperature profiles, and together with the developed quality model we can figure out which defrosting scheme is more optimal for product quality during refrigerated storage.

"In a later stage, we need also to take the overall energy consumption into our control objective. We plan to use the multiobjective optimization to design a new controller."

Optimization of a walk-in refrigeration system using R-404A was conducted by engineers from Tecumseh.

The researchers said, "Different parameter settings will affect system performance. Therefore, there is the need to investigate the effect of such behavior. The vapor-compression cycle is usually the refrigeration system of choice for walk-in types of applications.

"In order to optimize the refrigeration system design, understanding the system cycle is fundamental to understanding the behavior of the system's individual components as to choose the optimize points of operations. A walk-in refrigeration unit was constructed and installed onto a walk-in cooler box.

"The test unit is used for the investigation on the effect of different parameters towards system performance. The parameters investigated were condenser airflow, superheat setting, and refrigerant charge. These conditions were changed and their effects were discussed and recommendations of optimized conditions were presented."

Publication date: 09/04/2006