Lowering evaporator superheat (SH) can improve the efficiency of CO2(R744) refrigeration systems by up to 11.5%, according to Kurt Knapke, Vice President of Solutions Strategy for Cold Chain at Copeland, and Wynand Groenewald, Founder of CO2 refrigeration engineering consultant firm Future Green Now.
Knapke and Groenewald presented this finding during a presentation at the ATMOsphere (ATMO) America Summit 2024. ATMOsphere is the publisher of NaturalRefrigerants.com, and the ATMO America Summit 2024 was held June 10–11 in Washington, D.C.
The study, which lasted several months, involved analyzing 214 DOE/AHRI1200 certified display cases and 51 DOE/AWEF certified unit coolers under various coil temperature difference (TD) conditions. The display cases were analyzed with medium-temperature (MT) superheats of 6–8°F (-14.4 to -13.3°C) and low-temperature (LT) superheats of 3–5°F (-16.1 to -15°C), while the unit coolers were analyzed with a consistent superheat of 6.5°F (-14.2°C). The study’s goal was to evaluate different technologies and design approaches to optimize evaporator superheat levels and overall system performance.
“Our research shows that these methods can significantly improve system efficiency,” Knapke said.
The strategies
The study highlighted three primary strategies to enhance energy efficiency in CO2 refrigeration systems. These strategies were compared to a baseline system, which was designed using coil design superheat setpoints with an average coil TD. The three strategies involved using the highest coil TD, average TD and lowest TD. The annualized saving potential for these strategies was evaluated with MT conditions at 400MBH (117.2 kW/33.3TR) and LT conditions at 100MBH (29.3kW/8.3TR).
The three strategies consisted of:
- Energy modeled using no superheat with liquid ejector: Implementing liquid ejectors to maintain no superheat in the evaporators proved effective in increasing the saturated suction temperature and showed energy savings of 10.9% at a TD of 10°F (-12.2°C), 4.4% at a TD of 7°F (-13.9°C) and 3.2% at a TD of 4°F (-15.6°C).
- No superheat with liquid to low LT: Using liquid to maintain no superheat in low-temperature evaporators demonstrated increased saturated suction temperature and showed energy savings of 11.5% at a TD of 10°F, 5.1% at a TD of 7°F and 3.9% at a TD of 4°F.
- Dual suction with standard operating SH: The concept of dual suction lines, catering to different temperature requirements, presented energy savings of 7.2% at a TD of 10°F, 6.1% at a TD of 7°F and 6.3% at a TD of 4°F.
The research demonstrated that considerable energy savings could be achieved by selecting appropriate equipment and optimizing the system design. Using coils with the lowest TD for lowest-temperature load in the suction group resulted in energy savings without the need of advanced low-superheat technologies. Additionally, incorporating internal heat exchangers improved system performance by enabling operation based on design coil superheat rating points.
The implementation of liquid ejectors to maintain no superheat in evaporators increased the saturated suction temperature, leading to energy savings. Dual suction lines, which cater to different temperature requirements, also showed significant energy savings.
Practical implications
The study underscored the importance of selecting the right equipment and incorporating design elements to maximize efficiency.
“If you can reduce the superheat or get rid of your superheat, you can move the pinch point of your air temperature through your coil versus your refrigerant temperature, moving your pinch point to the most efficient location,” Groenewald explained. “That allows it to increase your saturated suction temperature, leading to energy efficiency.”
The findings suggest that integrating the study’s strategies with existing high-ambient climate solutions can further increase energy efficiency. By combining low SH technologies, dual-suction systems and optimal coil selection with other energy-saving measures, CO2 systems can achieve higher efficiency.
“We need to keep pushing the boundaries and exploring new solutions,” Knapke said.
“We need to keep pushing the boundaries and exploring new solutions.”
Kurt Knapke, Vice President of Solutions Strategy for Cold Chain at Copeland
