Now that CO2 refrigeration systems are becoming commonplace in supermarkets across North America, the question remains: What steps can be taken to make these systems more efficient?
A new study, backed by Copeland and Future Green Now (FGN), aimed to tackle that question head-on. The mission: Determine how tweaking the low side of a CO2 booster refrigeration system – specifically the suction superheat – could unlock annualized energy savings as high as 7.9%.
Establishing a Baseline
Since the efficiency of a refrigeration system is primarily influenced by the pressure differential that compressors must overcome, increasing the suction pressure should reduce the pressure differential and, consequently, lower the energy required to achieve the same cooling capacity.
To determine this, Copeland and FGN utilized modeling data from display cases using the highest, lowest, and average temperature differentials (TDs) across 214 display cases and 50-unit coolers representing five major original equipment manufacturers. Each case met current, applicable U.S. Department of Energy (DOE) and Air-Conditioning, Heating, and Refrigeration Institute (AHRI) 1200v specifications.
To establish the impact of the evaporator coil TD on the lowest temperature load, a baseline system was established. The system assumptions and baseline performance data are shown in Sidebar 1 and Table 1. This information notes the highest TDs for the tested display cases and unit coolers. Also noted are the discharge air temperatures for each product, suction line losses, and the compressor saturated suction temperature (SST) based on various coil TDs for the lowest temperature loads.
SIDEBAR 1: Baseline System Assumptions
The baseline system was a typical CO2 booster system with no high-side ambient system optimizations. Here is its measured performance data:
- Medium temperature (MT) load: 400MBH
- Low temperature (LT) load: 100MBH
- Suction line losses: 2°F for both MT and LT
- Software: Engineering Equation Solver (EES)
- Gas cooler standard operating conditions: 59°F minimum saturated condensing temperature (SCT); 14°F temperature differential (TD) subcritical (SC); and 6°F TD transcritical (TC)
- Weather data: National Renewable Energy Laboratory’s typical model year (TMY3)
- Sample cities: Jacksonville, Florida; Chicago; and San Jose, California
TABLE 1: Performance data for the baseline CO2 refrigeration booster system.
| Product Type | Type of Coil | Air-Off Temperature Range (F) | Suction Line Losses (F) | Highest Coil TD | Compressor SST (F) | Lowest Coil TD (F) | Compressor SST (F) | Average Coil TD (F) | Compressor SST (F) |
| Meat/Deli | Display Cabinets | 30 | 2 | 10 | 18 | 4 | 24 | 6 | 22 |
| Cold Room | Unit Coolers | 34 | 2 | 10 | 22 | 10 | 22 | 10 | 22 |
| Beverage/Produce | Display Cabinets | 38 | 2 | 8 | 28 | 4 | 32 | 6 | 30 |
| Frozen Food | Display Cabinets | -6 | 2 | 10 | -18 | 4 | -12 | 7 | -15 |
| Frozen Holding Room | Unit Coolers | -4 | 2 | 10 | -16 | 10 | -16 | 10 | -16 |
| Ice Cream | Display Cabinets | -13 | 2 | 10 | -25 | 4 | -19 | 7 | -22 |
The Correlation Between Suction Pressure and Pressure Differential
Once the baselines were established, the team began experimenting with different parameters to test their hypotheses.
“We wanted to get as close as possible to the optimal superheat these coils were designed to operate at,” said Andre Patenaude, CET, director, solution strategy, Copeland. “The industry standard in North America is 10°F for all low and medium temperature evaporators.
In our optimal system, we used 5°F superheat, as we wanted to design a test system that performed more efficiently.”
Though the study, the testers utilized the lowest coil TD possible, a 4°F instead of a 10°F, to see what would happen.
“When you go from the highest coil TD to the lowest, there’s a big jump in the suction pressure, and that’s where your true efficiency gain is,” said Patenaude.
For example, a meat/deli case with a discharge temperature requirement of 30°F and a maximum TD of 10°F necessitates the entire suction group operates at a compressor SST of 18°F at 394 psig. If the meat/deli cases had a TD of 6°F instead of 10°F, the SST would increase to 22°F (420 psig). This change would result in a 26 psig increase in pressure, leading to a 1.5% reduction in compressor power and an 8% increase in compressor capacity.
“By changing the evaporator design, the system is increased to 22°F on the lowest temperature load,” said Patenaude. “That increases the psig to 420, which reduces the compression ratio, the total heat rejection, and the stress on the condenser. Additionally, the compressor uses less energy because the compression ratios have been decreased. The same thing happens on the low-temperature side, resulting in a 23-psig increase.”
Generally speaking, the TD determines the SST required to maintain the specific air temperature.
“If the evaporator is designed for 10°F TD, it’s going to give you the actual discharge air required with that 10°F TD,” said Patenaude. “If it has a 4°F TD, it will require a different design to provide that same discharge air temperature. Thus, a lower TD results in a higher SST.”
Significant Energy Savings
When comparing the baseline system to one using the TD of around 6°F, the results were astonishing.
The lowest SST on the medium temperature side of the baseline system is 22°F common suction. On the low side, that number is minus 22°F.
Reducing the ice cream configuration from a 10°F TD to a 7°F TD and the meat cooler from 10°F to 6°F results in a 6.8% annualized savings. On the low-temperature side, with a 4°F TD, the common suction SST is decreased from minus 22°F to minus 19°F, resulting in an additional 1.1% savings — a total annualized savings of 7.9%.
“On the medium temperature side, that’s a 26 psig increase in suction pressure, and, on the low-temperature side, that’s a 23 psig increase,” said Patenaude. “Now, even the low-temperature compressor’s total heat rejection is decreased, and the power used is less because the compression rations have been reduced. This results in a lesser load on the medium temperature compressors, resulting in an additional 1.1% annual savings.”
This study opens a critical conversation for grocery retailers, refrigeration engineers, and sustainability teams alike. If you’re already using CO₂ systems to shrink your carbon footprint, it might be time to look deeper — into the low side — for even bigger savings.