A Publix store in central Florida using a transcritical CO2 (R744) system currently shows a 7% premium to reach energy parity with an HFC system, while stores further north in cooler climates show no difference or even energy savings from switching to CO2.
This analysis was presented by Doug Milu, Manager of the Refrigeration & Energy Program at Southeastern U.S. retailer Publix Super Markets, and Derek Gosselin, Director of Technical Product Support of U.S. OEM Hillphoenix, during the FMI (Food Industry Association) Energy and Store Development Conference in Orlando, Florida, on September 19–21.
Milu and Gosselin explained how Publix worked with Hillphoenix to develop an energy analysis tool to gauge how various measures are closing the parity gap between an HFC and CO2 system in a warm climate like Florida.
When it comes to reaching energy parity, Milu noted that “we haven’t gotten there yet, but we are doggone close.” The system is still a work in progress with various tweaks still underway to close the parity gap.
Publix is very happy with the CO2 systems installed in Florida. “I need the design to be reliable, repeatable, scalable, sustainable, serviceable and redundant, and we get that with this system,” said Milu.
The energy analysis that found the 7% energy difference was conducted at Publix’s first transcritical CO2 system at a new store in Longwood, Florida, near Orlando, which opened in June 2020. It features two Hillphoenix racks and Danfoss controls.
Optimizing the controls and drives played a big role in enhancing the energy efficiency of the system, as did adding an adiabatic gas cooler and parallel compression. Publix plans to use ejectors in future stores while other technologies include subcooling and pressure exchangers.
Hillphoenix developed an Energy Calculation Tool to compare CO2-based and HFC-based systems. The OEM tested the tool by hooking up a datalogger throughout the entire CO2 system at the Longwood store.
To validate the tool, the Florida store’s energy data was captured for a year. When comparing the calculated data from the tool versus the actual energy data captured on a monthly basis, the difference was at most 1.73%, with an average difference of 0.39%.
Gosselin and Milu showed a U.S. map with all of Publix’s store locations and the calculated energy differences between transcritical CO2 stores like the one in Longwood and R449A DX stores. The difference ranged from an energy premium for CO2 of 8% to 6% in south Florida to an energy savings of 6% to 8% in Virginia. “CO2 booster systems with the right technology applied can achieve energy parity while providing the right long-term sustainable option to support a net-zero goal,” said Gosselin.
Publix has 1,300 stores with 15 different prototypes (six with CO2). This meant the chain could easily identify an HFC store with a similar load profile as the CO2 store in Longwood, ensuring that the data was comparable. Normalized weather data was used in the comparison.
The energy calculation was done without taking into account the additional benefit of heat reclaim (which reduces natural gas usage). CO2 systems have far better heat reclaim potential than HFC systems, and if this is considered, the 7% disparity could be reduced further to as low as 4.5%, said Milu. Data was shown indicating that a store in Miami with a transcritical CO2 system would use 20% less natural gas than a store with an R448A system.
It is important to look beyond just the impact of the refrigeration system to save energy, Gosselin advised. When considering the impact of the total store’s operation, it’s key to consider the case as well. Using a case controller and electronic expansion valves contributed an estimated 8.7% improvement in case energy input versus mechanical valves, Gosselin noted.
Retrofitting with CO2
Publix is currently retrofitting a store in Atlanta from HFC to CO2 with the help of Hillphoenix. Completion is expected in Q1 of 2023. Three AdvansorFlex-Mini CO2 units are being installed, providing low-temperature, ultra-low-temperature and medium-temperature refrigeration. The HFC system is being replaced in stages, and thanks to the phased remodeling, the store does not have to suffer any downtime.
Typically, CO2 systems have a smaller footprint (smaller compressors) and can easily fit into an HFC machine room, explained Gosselin.
Milu expects CO2 systems to be the norm for the retail sector within the next 10–20 years. That is why Publix is moving in this direction and will continue to roll out CO2 systems in its stores. By the end of 2022, Publix planned to have eight stores with transcritical CO2, and more than 100 of its stores have some form of CO2 – transcritical, cascade or liquid overfeed.
‘It’s not that difficult’
Milu highlighted the importance of training and getting all contractors up to date with CO2 and how to work with it. His message to contractors is that CO2 is not that different to what they have worked on throughout their whole career.
“There is nothing to be afraid of,” said Milu. “It’s just a refrigeration system. Understand the nuances of working with these systems, and let’s get on with it. It’s not that difficult. I’m here to tell you, after more than 100 sites, it’s not that big of a deal. But you have to pay attention. Training is key.”