Comparing COPs

Researchers from the University of Sheffield in England and the British University in Dubai compared the coefficient of performance (COP) for maximum optimization of beverage coolers. The focus was on the dry condenser fans in refrigeration systems. The summary said, “The simple refrigeration system used for this investigation was based on a commercially available (HFC) 404A/CO2 system comprising the basic components, with the condenser having extractor fans. The results show that, when the outdoor temperature is below about 15°C (59°F), there is no observable difference between these two approaches. However, when the ambient temperature increases beyond this threshold, the control method which optimizes COSPis significantly better for part load conditions. This indicates that maximizing the COPcan lead to a sub-optimal system in terms of energy consumption under part load conditions. When the refrigeration system is at its full load point, however, both approaches produce similar results again.”

CO2 in Wide Range of Applications

Looking at ways CO2 can be used as a refrigerant in a wider range of applications was a topic involving waste heat dehumidification from researchers at the Oak Ridge National Laboratory. They used an energy modeling tool called EnergyPlus to “to investigate the energy consumption of supermarket utilizing packaged rooftop air conditioning units and a transcritical CO2 booster refrigeration system.” Among the findings: “In general, the waste heat from the transcritical CO2 booster system satisfies more of the heating requirements of the supermarket than does that of the R-404A multiplex DX system. For the waste heat recovery scheme, it was found that on average, the CO2 booster system could satisfy 65 percent of the supermarket’s total heating needs while the R-404A multiplex system could only meet 54 percent of the heating requirement.

“From the analysis presented, it appears that the waste heat recovery scheme (i.e., desiccant regeneration, water heating and space heating) for transcritical CO2 booster refrigeration systems has merit. As a next step, the results of this analysis could be validated through experimentation by designing a lab-scale implementation of the proposed scheme, whereby the proper sizing and control of the waste heat recovery components are determined.”

Reducing Charges

“Refrigerant Charge Reduction in Small Commercial Refrigeration Systems” was a topic of research being done at the University of Illinois. A bottle cooler was used. The authors of the paper presented at Purdue said, “Most of the charge is retained in the condenser and liquid line, while a small portion of charge is retained in the evaporator and compressor. The condenser contains two-phase and subcooled refrigerant, and the liquid line contains subcooled refrigerant, which lead to a large amount of charge retention. Based on the model prediction, flattening the finless-round-tube of the heat exchanger to some proper extents is a simple way to reduce charge without penalizing the system performance.”

Transcritical CO2

Additional research at the University of Illinois focused on use of CO2 (R-744) in a transcritical system to provide cooling to a beverage display unit as compared with a unit using HFC-134a.

In summary, the paper said, “This study demonstrates that it is possible to design high-performance transcritical R-744 glass door merchandiser systems that deliver cooling capacities and energy efficiencies that are comparable to results obtained with an R-134a baseline system. In the present case, pull down time of the improved R-744 was slightly lower (97 percent), while energy efficiency was almost equal (103 percent) for tests conducted at an elevated ambient temperature of 32°C (89.5°F) and 65 percent relative humidity. The most important observation is that these promising system results were obtained with low-cost technology, which was very comparable to what was used in the R-134a system: round-tube-plate-fin gas cooler and evaporator, capillary tube, and a fixed speed compressor. The improvements were realized by optimization of gas cooler circuiting and a rigorous optimization of refrigerant charge and capillary tube geometry.”

Propane in the Equation

Yet a third paper from the University of Illinois at the same session — this one done in conjunction with Creative Thermal Solutions — looked at beverage display coolers running on propane. It was the contention of the report that “the majority of beverage manufacturers prefer natural refrigerants over synthetic refrigerant options as the working fluid for their beverage display cooling equipment.” The purpose of the research was to determine how to make propane (R-290) work in extremely low refrigerant charges.

In part it said, “The modeled propane charge in the compressor of 38 grams showed that a significant amount of refrigerant was absorbed in the oil. Therefore to reduce the system charge, the amount of oil in the compressor was decreased. This led to an estimated 21g of refrigerant charge absorbed in the oil in the compressor. Based on the compressor estimation, multiple condenser designs were studied.

“A microchannel condenser design was selected due to its small volume that minimizes the charge in the component. The condenser was designed to split into two parallel sections on the refrigerant side to reduce pressure drop. A low fin density was used to reduce possibility of fouling.

“Other system improvements were incorporated on the cooler with a total capacity of 700 cans with 355 ml of carbonated soft drink each. These improvements included reducing the tube internal diameter in the liquid line, removing the accumulator and filter drier, and installing an internal heat exchanger by using the capillary tube wrapped around the evaporator return line.

“The cooler was validated at 32.2°C (89.96°F) and 65 percent relative humidity. The allowed pull down time to reach product temperatures between 0°C (32°F) and 7°C (44.6°F) was 19 hours. The system achieved a pull down time of only 17 hours and 30 minutes. Therefore the system passes this requirement in the standard.

“The results show that for a system with cooling capacity of approximately 1 kW, a highly efficient bottle cooler can be designed to have total R-290 refrigerant charge of 50g or less.”