CO₂ as a refrigerant, commonly referred to as R-744, dates back to the late 1800s. Used for decades, the onset of chlorofluorocarbons (CFCs) essentially eliminated it from the market in the 1940s.

As environmental concerns about CFCs and hydrochlorofluorocarbons (HCFCs) surfaced in the 1990s, R-744 — boasting an ozone depleting potential (ODP) of zero and a global warming potential (GWP) of 1 — was reintroduced as an environmentally safer alternative. Since then, the refrigerant has gained a foothold in Europe. Per industry data, more than 40,000 CO₂ refrigeration systems have been installed there. On the flipside, the U.S. is just getting up to speed, as fewer than 1,000 systems have been installed domestically.

Most recently, a pending global hydrofluorocarbon (HFC) phasedown and additional efficiency regulations have bolstered R-744’s position in the American food retail market, further accelerating interest in the refrigerant’s unique benefits.

Introducing CO₂

CO₂ refrigeration systems feature numerous unique characteristics, including:

• Its evaporation latent heat is large, and the refrigerating capacity per volume is about five to eight times that of traditional refrigerants;
• The kinematic viscosity of CO₂ is very low and can remain so at low temperatures;
• The thermal conductivity of CO₂ is high, and the ratio of liquid density to vapor density is low. After throttling, the refrigerant can be distributed evenly throughout refrigeration pipes;
• Accelerated flow and heat transfer performances of CO₂ result in smaller compressor and refrigeration equipment, helping to minimize the system’s operational footprint; and
• The chemical properties of CO₂ are stable; and
• The refrigerant is nontoxic, harmless, noncombustible, and will not decompose into toxic gases at high temperatures.

As R-744 becomes more desirable to cold case operators, many HVAC contractors are just now acclimating themselves with the gas and its nuances.

Adam Dykstra, refrigeration operations manager, DeTroye Electric Service, Oostburg, Wisconsin, is among them. Before ever touching a CO₂ system, DeTroye’s team trained with a manufacturing partner to learn the basic ins and outs of the equipment.

“From a pressure standpoint, we were going from systems with 20 pounds of suction to systems that run at 300 pounds of suction,” he said. “The high head pressure of these systems increased from 400 to 1,800. There was definitely a learning curve when it came to these attributes.

Traditionally, DeTroye’s techs have always utilized their own controls for their grocery store customers. The team had to spend some time to acclimate itself with new system controls as well.

“We had to learn how they work and how they’re programed,” said Dykstra. “Some of the stuff they’re doing with superheat is vitally important, so we had to learn how they work and teach that to our team.”

In a very short time span, DeTroye has completed two CO₂ systems — a storage warehouse and grocery store.

“We’ve come to appreciate CO₂ systems and the numerous benefits they offer — they’re smaller, and the cost of ownership tends to be lower,” said Dykstra. “Additionally, there currently are no maintenance regulations. While ammonia systems require specific preventive maintenance and quarterly reports, those requirements don’t exist with CO₂.”

As the demand continues to escalate, the supply is following suit. The refrigerant is becoming more prevalent, making service and maintenance much more convenient.

“In just the last few months, it’s become much more affordable and accessible,” Dykstra said. “We’re now able to keep a charge on hand, which is important because if a customer goes down over a weekend, we want to be able to bring them back online as quickly and efficiently as possible.”

Peter Savage, project manager, controls division, AAA Refrigeration Service Inc., said his company installed its first CO₂ system in 2015. His techs were initially concerned about the system’s increased pressure, as CO₂ systems can reach pressures as high as 1,600 psig on the high side of a transcritical system.

“We’ve dealt with high-pressure refrigerants before,” he said. “R-22 ran at about 150 pounds of pressure, and the move to R-410A increased that pressure to about 400 pounds. I was confident we’d be able to adapt, but we had to learn on the fly.”

To prepare, Savage said his team trained heavily on R-744’s unique characteristics and high pressure demands.

“We connected with a reputable refrigerant manufacturer and invested in a lot of training,” he said. “We’ve kept up with that training and regularly bring those trainers back to our location to discuss new technologies and innovations.”

Over the last eight years, AAA Refrigeration Service has been involved in approximately 20 CO₂ projects. Overall, Savage said he is a fan of transcritical CO₂ systems and sees an added benefit for urban, multi-use projects due to the equipment’s reduced line sizes.

“We've built out some smaller supermarket equipment, and our piping installers love these jobs because the piping is smaller and lighter,” said Savage. “Instead of running out 2- to 2 1/2-inch loops, you’re running out one or two 1 5/8-inch loops. This may not seem like much, but every little bit matters when you’re dealing with space constraints.”

Overall, Savage predicts CO₂ will be a major player in the HVACR space moving forward.

“CO₂ is a great option already, but the future is bright,” he said. “As manufacturers continue to explore smaller applications, I believe there may be a place for a CO₂ heat pump package unit or a smaller CO₂ single unit for a small convenience store. I expect the marketplace for CO₂ to continue to evolve in-step with technology.”