Comparative Field Assessment of CO₂ System Performance vs. HFC Refrigeration in Supermarkets

Executive Summary Supermarket refrigeration accounts for nearly half of the electricity consumption in grocery stores, making it one of the largest opportunities for both energy efficiency improvements and greenhouse gas reduction. In California, recent regulations now require new supermarket refrigeration systems with a refrigerant charge greater than 50 pounds to use refrigerants with a global warming potential below 150. This has accelerated the transition away from high-global-warming-potential hydrofluorocarbons, such as R-404A, toward low-global warming potential alternatives, including carbon dioxide (R-744).Despite strong regulatory drivers, important questions remain about energy performance, operational reliability, and the cost-effectiveness of carbon dioxide systems across California’s diverse climate zones. To help address the knowledge gap, this project, a CalNEXT field study (ET25SWE0005), compared two carbon dioxide and two hydrofluorocarbon supermarket refrigeration systems located in Climate Zones 10 and 11.ApproachThe project team installed calibrated three-phase power monitoring and temperature logging equipment to collect detailed performance data over a four-month period. Using this data, the team developed regression models to correlate refrigeration system energy use with outdoor temperature. Annualized results were produced by applying the models to historical weather data. In parallel, the team’s on-site engagement with store personnel and corporate stakeholders provided insights into operational practices, perceived risks, and market adoption considerations.KEY FINDINGSEnergy: After adjusting for system load and equipment efficiency, the carbon dioxide systems showed little to no net energy savings compared with hydrofluorocarbon systems in these warmer climate zones. Above ~95°F, carbon dioxide system efficiency can diminish due to transcritical operation, limiting performance gains.Economic: Installed costs for carbon dioxide systems were approximately 30 percent higher than for hydrofluorocarbon systems. With minimal avoided energy, simple payback calculations were unfavorable when based solely on energy savings.Environmental: While the carbon dioxide systems experienced 7 to 19 percent annual leak rates, their climate impact was minimal (global warming potential of 1). By contrast, even modest leaks of R-404A would generate substantial carbon dioxide emissions, given its global warming potential of 3,922.Operational feedback: Store personnel at carbon dioxide sites described the two systems assessed as more complex, sensitive, and less reliable to restart after outages. One location experienced a major outage caused by a high-pressure leak and associated product loss. Hydrofluorocarbon systems, by contrast, were viewed as reliable and easy to restart.Stakeholders: The grocery store chain’s corporate refrigeration management emphasized that, despite uncertain return on investment, compliance with California regulations and alignment with company sustainability goals are the primary drivers for carbon dioxide refrigeration system adoption. However, field-validated performance data were consistently highlighted as essential for informing future capital investment and incentive participation.Conclusions This limited field assessment suggests that in warmer California climates, adoption of carbon dioxide refrigeration systems may not be justified by direct energy or cost savings alone. Instead, regulatory compliance, corporate greenhouse gas reduction commitments, and potential utility incentives are the leading motivators. The findings underscore the need for broader field studies across additional climate zones and store configurations to provide a more representative dataset on energy performance, leak rates, and operational reliability.By producing objective, field-based evidence, this study provides information to inform Investor-owned utility program design, policy alignment, and industry decision-making as California advances its transition to low-global-warming-potential refrigeration.Recommendations Based on the findings from this limited field assessment, the project team suggests the following recommendations to assist in future utility research, program design, and market development efforts:1.Expand Field Data Across More Climate Zones and Store TypesConduct additional monitoring of CO₂ refrigeration systems in cooler and coastal California climate zones (CZ 3–6) as well as in larger-format and small-footprint stores. Broader datasets will enable more representative comparisons of energy use, leak performance, and reliability.2.Develop Targeted Incentive FrameworksBecause CO₂ system adoption is primarily compliance- and sustainability-driven rather than energy-cost-driven, consider incentive approaches that reflect carbon reduction value, refrigerant avoidance benefits, and lifecycle emissions rather than only kWh savings.3.Support Workforce Training and Commissioning Best PracticesThe higher leak rates and restart challenges observe in this assessment, suggest a need for enhanced technician training, standardized commissioning protocols, and expanded service infrastructure to ensure reliable operation and lower maintenance costs.4.Encourage Manufacturer and End Use Customer CollaborationUtility programs and CalNEXT can convene online presentations and discussion for manufacturers, OEMs, and retailers to share performance and reliability data, identify common failure modes, and review potential system designs for transcritical CO₂ operation in warm climates.5.Continue Market Transformation ResearchTrack CO₂ refrigeration deployment costs, leak rates, and energy trends over time as the market matures. Longitudinal studies will help quantify cost reductions, performance improvements, and carbon benefits as systems evolve.6.Integrate CO₂ Findings into Future Emerging Technology and Custom Program PathwaysIncorporate these field results into the CalNEXT Technology Priority Map and custom measure development to support consistent evaluation criteria for low-GWP refrigeration technologies.