One of the most closely watched stores in the supermarket industry in the U.S. is one operated by Sprouts Farmers Market in Dunwoody, Georgia; a 29,000-square-foot facility (2,694m2) opened in July 2014.
This is the first store in the U.S. to test whether an all-CO2 transcritical refrigeration system can operate efficiently in a warm, southern climate, whose ambient temperatures often rise above 88°F (31°C).
Above that “critical point” – known as supercritical mode – the CO2 gas does not condense in the condenser (which instead serves as a gas cooler) and does not become a liquid until after the pressure is reduced.
Transcritical system outperforms high-GWP R407A
At last year’s ATMOsphere America conference, Jeff Newel, director of research and development, Hillphoenix, presented an analysis of the expected energy consumption of the Dunwoody store’s transcritical system compared with that of a DX R407A system at a comparable store.
The transcritical system was projected to be 6.3% more efficient that the R407A system. With almost a year’s worth of actual data, Newell demonstrated at this year’s conference that the projected analysis was accurate – despite the warm climate, the transcritical system was more efficient, duplicating the performance of transcritical systems in northern climates.
“The 2014 theoretical comparison is correct; the energy consumed is less than the standard system,” said Newel.
In addition, the store’s energy efficiency, combined with eliminating HFCs and reducing carbon emissions, earned it the EPA GreenChill program’s platinum-level certification for two consecutive years.
The Sprouts Dunwoody store compensates for the higher ambient temperature by employing an adiabatic condenser/gas cooler rather than a conventional condenser.
“The lower you can get that temperature on the returning CO2 from the gas cooler, the better,” said Newel. “The adiabatic gas cooler gives you the ability to do that.”
Newel explained that the adiabatic gas cooler operates dry in cooler weather, but in warmer weather fills with water, which evaporates and cools the air to about 50°F (10°C).
Hillphoenix is testing ejector and parallel compressor technologies that can also be used in transcritical systems to compensate for higher ambient temperatures.
These technologies can “get the same or possibly better performance” than the adiabatic gas cooler, said Newel. The decision of which to choose will rest on initial costs.
Following Newel’s presentation, Reggie O’Donoghue, director of product management for Emerson Climate Technologies, explained the advantages of using electronic controls in CO2 transcritical booster systems like the Sprouts store in Georgia, as opposed to mechanical controls.
Electronic controls are designed to manage the complexities of a transcritical CO2 system as the gas cooler toggles between subcritical and supercritical mode.
“It’s a no brainer – electronic controls are better for transcritical CO2,” he said.
“Mechanical controls won’t be as efficient, especially in Georgia.”