HFCs such as R-404A and R-507 are commonly used in supermarket refrigeration systems, but these refrigerants will likely become more difficult to obtain in the future, as they are being phased down under the AIM Act. As a result, supermarkets are looking at replacing HFCs with other so-called ‘natural’ options that have zero or near-zero GWP, such as ammonia, CO₂, and propane.

Helping to guide supermarket owners and operators through the transition process is the North American Sustainable Refrigeration Council (NASRC), which recently held its Sustainable Refrigeration Summit. The virtual event featured sessions covering everything from regulations to new technologies to utility incentives. In an informative session, entitled “CO₂ Systems: What Retailers Need to Know,” grocers shared the pros and cons of incorporating CO₂ refrigeration systems into their stores.

Moving to CO₂

The number of supermarkets installing CO₂ (R-744) systems around the U.S. continues to grow. Publix — which operates over 1,300 supermarkets in seven southeastern states — and Coborn’s — which operates 66 grocery stores in five midwestern states — are two of the grocery chains that have installed the new technology.

One of the reasons why Coborn’s chose CO₂ instead of a ‘synthetic’ GWP refrigerant is that the company wanted to make sure that any alternative it selected would be future proof, said Chris Braun, senior project manager of construction, refrigeration, and facility maintenance at Coborn’s.

“We wanted to make sure when we were spending the capital to do a new store or a remodeled store, that we were setting ourselves up for the next 20 years-plus — that we weren't going to put something in and have it be regulated out in the next five to 10 years,” he said. “We also wanted to make sure that we had contractors in the area who could support the technology and that we could get it installed for a cost-effective manner. It also needed to be financially viable in upfront cost — and so we could justify it with the total cost of ownership of the equipment over that lifespan of the equipment.”

Corbon’s installed its first transcritical CO₂ system in a new store that was built in 2020 and has since installed transcritical CO₂ systems in both another new store and a remodeled existing store. The company uses adiabatic gas coolers on all sites and plans to use transcritical CO₂ systems in all new stores going forward, said Braun.

“We also have plans to remodel a couple of existing stores in 2023 and 2024, where we’ll take out HFC refrigerants and put in transcritical CO₂ systems,” he said. “That's due to the fact that we’re seeing utility savings with CO₂ versus standard HFCs.”

Publix installed its first CO₂ system in 2010, which was a low-temperature cascade system, said Doug Milu, refrigeration and energy program manager at Publix. The company now has 100 stores operating with CO₂ as a primary refrigerant.

“We started with the low-temp, cascade, CO₂ systems, and those were really key in our decision to move forward, because we learned about handling CO₂, working through contractor issues, CO₂, startup, and subsequent operational issues working with the CO₂ systems,” he said. “The majority of our systems are referred to as liquid overfeed, so we have low-temp CO₂ DX refrigeration systems, and we're pumping 20°F liquid CO₂ throughout the entire medium-temp cases that are on the sales floor. We now have six transcritical CO₂ systems in operation, and CO₂ is really becoming more of a standard for new stores.”

Advantages

Some of the advantages of using CO₂ include smaller loop piping and system architecture that is considerably smaller than standard HFC systems, said Braun. As a result, Coburn’s has been able to reduce the size of their mechanical rooms, as well as save money on structural steel, as there is only one rooftop gas cooler, rather than multiple fan condensers.

“We also use CO₂-to-glycol heat exchangers, so we can use the glycol for vestibule in-floor heat and snow melt in lieu of a natural gas boiler in winter months, and we're able to eliminate the cost of that boiler altogether,” he said. “We use the same heat exchanger for a glycol-to-air coil to aid with dehumidification in the summer.”

Another advantage the CO₂ systems provide is significant energy savings. Braun said he expected to see anywhere from a 5% to 10% energy savings with CO₂ compared to HFCs, but the stores are actually achieving much better efficiency. In the new store, they are seeing an average of 15% utility savings per square foot with the CO₂ system, compared to R-448A, while the remodeled store saw even bigger savings.

“Our remodeled store went from six systems scattered throughout the store, some of which were built in 1982, to a new transcritical CO₂ system with all new cases, and we're seeing 25% energy utility savings in that store,” said Bruan. “I'm actually probably being a bit conservative on that, because we've only got nine months’ worth of data on that store. We are seeing an increase in water usage due to the adiabatic gas coolers up there, which equals to about $200 a month in the summer, but natural gas usage is down about 2% to 3%.”

In warmer climates, such as Florida, the energy savings may not be as significant. Milu noted that Publix is averaging about 4% higher energy use with CO₂ systems than with traditional HFC stores of similar size and location.

“In central Florida, we’re going to pay a little bit of an energy penalty for these systems, because we don't get the relief of cooler temperatures in these areas very often,” he said. “As you go north, you can gain some really significant energy savings. But I also want to stress optimizing the systems after startup — commissioning, making sure that the systems are running correctly, that the drives are dialed in. With the right technology, you can not only reach energy parity, but you can save some energy as well, based on where the store is located.”

Lessons Learned

With CO₂ systems, there is usually a bit of a learning curve for both the grocer and the installing contractor, which is why Milu recommends that grocers work closely with their system supplier and controls provider, as well as engage early with the installing contractor to help ensure a smooth construction process. There are also other issues that grocers (and contractors) should know about CO₂ systems.

“We still have a problem with the availability of refrigerant-grade CO₂,” said Milu. “Because there's a gap in the supply chain for CO₂ to be readily available in some of our operational areas, we are storing CO₂ in those locations. During startup, this is probably not an issue, but if it’s an existing store, you really don't want to wait three days before you can get a delivery of CO₂.”

Publix has also decided to move forward with more transcritical systems, so there is a need to work on the total cost of ownership, said Milu.

“Our CO₂, low-temp cascade and liquid overfeed systems are fairly dialed in. They work really, really well, and they have reached energy parity,” he said. “But they have pigeonholed us into a one rack system for the entire low- and medium-temp systems, so there is still a redundancy issue. If I have a problem going forward, I put everything at risk at the store. So that's why we have decided to move towards the transcritical systems, because we can divide the low- and medium-temp systems within a couple of different racks, which then helps us with the redundancy issue.”

Braun pointed out that using a transcritical CO₂ system in his northern climate makes sense, as the system only goes supercritical 12 to 14 days a year. When that happens, though, the system does seem to have more issues.

“Oil issues, temperature issues, higher energy — just things like that. We do see a lot more service calls when it goes supercritical,” he said. “A cooler climate and an adiabatic condenser can keep high ambient conditions to a minimum.”

They are also seeing an overall increased amount of service calls, said Braun, which is often due to the case controllers that are typically not installed on HFC systems. Service calls also tend to be more intensive, and the leak rate is considerably higher than HFC systems. Part of that is due to CO₂ lost during service calls, if the shutdown is not done properly. In addition, there are longer restart times after power failures.

“Typically if you have a power failure, you can start that system up and in 30 minutes you're up and running,” he said. “With CO₂ systems, you have to stage these things on, bringing case controllers on in blocks, so a two-hour power outage could turn out to be four hours before the last system starts staging on. There is also the CO₂ shortage that Doug talked about earlier, so we do keep a full store charge on site at all three sites. Not only is the CO₂ cylinder availability a problem, but you're also paying a monthly lease on them, which does add up.”

Both Braun and Milu stressed the need for finding quality refrigeration contractors who can properly install and service CO₂ systems. Publix is leaving nothing to chance, having created the Publix Refrigeration University to help contractors and technicians get up to speed on working on CO₂ systems.

“Our training center has a full spectrum of whatever equipment you would see in our new stores moving forward and some of our existing stores that need to be serviced,” said Milu. “I can run failure scenarios on every component that's in the machine center and in the case. We can identify and simulate all the critical failure points that would happen out in a live store and have the technicians go through and verify that they understand not only operational sequence and control strategy, but the electrical components and communication wiring that are managing all of the functions of the system. That's a huge game changer.”