Ask the Expert: Is a CO2 Refrigeration System a Good Option?

Question: When is a CO2 Refrigeration System a Good Option for a Supermarket/Convenience Store?

Bryan Kilgore, ERS, for Zondits

The cost margins for retail food stores are razor thin, so when does upgrading or replacing an old refrigeration system with a new CO2 system make sense? Will the system be efficient, reliable, safe, and cost-effective?
Answer: There are a few key characteristics to consider when deciding if a CO2 refrigeration system is a good option for your store: safety, cost, efficiency, and environmental effects.
The two primary safety concerns are high operating pressures and CO2 refrigerant leaks. These risks can be mitigated however, because system components are designed for high pressures and include adequate leak detection and ventilating systems.
The cost of retrofitting to a CO2 system can be broken into equipment costs, installation costs, and operations and maintenance (O&M) costs. In general, the equipment tends to be more expensive than a traditional hydrofluorocarbon (HFC) system (the most common refrigerant used today), but this is typically offset by a decrease in installation and O&M costs. Due to the high-pressure operation and physical properties of CO2, the copper piping can have a smaller diameter, reducing installation costs. Overall, CO2 is a significantly cheaper refrigerant compared to HFCs.
Transcritical CO2 system efficiency is more sensitive to outside air temperature than traditional HFC systems; the transcritical CO2 system is more efficient when outside air temperatures are less than 80°F and typically less efficient when outside temperatures are greater than 80°F. However, there are ways to increase the efficiency of the system year-round, such as reclaiming heat from a gas cooler for hot water heating. Both cascade CO2 systems and secondary loop CO2 systems do not have the same environmental benefits that 100% CO2 transcritical systems do. However, both systems do not have the efficiency penalty to operate with supercritical CO2. Secondary loop CO2 systems do require additional power due to pumping liquid CO2 to end uses, but this addition still uses less power than a chilled glycol secondary loop.
The primary benefit of using CO2 refrigeration is environmental: CO2 has no ozone depletion potential. And while CO2 is itself a contributor to global warming, CO2 has thousands of times less global warming potential than HFC refrigerants. The impact of refrigerants and global warming comes in two forms, from electricity consumption and refrigerant leaks. Although refrigerant leaks are not a negligible source of greenhouse gas emissions, electricity consumption makes up the vast majority of the global warming impact of refrigerant systems, due to the CO2 emissions from utility thermal power plants.
Market Availability
There are many different CO2 systems already on the market today. Europe and Asia were early to embrace the revitalized CO2 refrigeration in supermarkets and convenience stores and the market is still growing. And the CO2 refrigeration market in Japan for small-format stores is undergoing significant growth. Some of the newest and most compact forms of CO2 refrigeration systems designed for small-format stores are available for the Europe and Asia markets but not yet available in North America. But the North American market for larger-scale CO2 refrigeration is growing steadily with supermarkets such as Hannaford and DeCicco & Sons, among others, switching to CO2 refrigeration systems.
In the end, the driving factors to switch to a CO2 refrigeration system are the sustainability and environmental benefits of removing HFCs from a store, and energy savings and O&M cost savings are also possible under the right circumstances.
For more information on new and emerging refrigeration technologies, see The State of Technology: Refrigeration.

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