Walk-in Coolers/Freezers low GWP and EE performance assessment (SCE-CARB-CTS collaboration)

This study was conducted to investigate the benefits and demonstrate the technical availability of transcritical carbon dioxide (CO2) technology for a walk-in refrigeration application. It is well known that various Hydrofluorocarbon (HFC)-based refrigerants in refrigeration end uses, such as grocery stores, can be environmentally adverse. There are existing and planned state, local, and federal regulations to promote alternative refrigerants with lower Global Warming Potential (GWP) and Ozone Depletion Potential (ODP). For this project, we developed and tested a demonstration medium-temperature CO2 walk-in cooler. The results of comparing the tested capacity and efficiency between the CO2 unit and the R448A baseline unit demonstrated transcritical CO2 is technically ready for walk-in applications, with no less performance than the baseline at comparable complexity and cost levels. A matched pair of cooler and condensing unit walk-in coolers with a rated capacity of 6,000 Btu/h was selected from the market as the baseline unit. Both units were tested individually for their Annual Walk-In Energy Factor (AWEF) ratings per Air-Conditioning, Heating, and Refrigeration Institute (AHRI) Standard 1250-2020, to verify compliance with Department of Energy (DOE) Energy Efficiency (EE) requirements. Since units using CO2 refrigerant are excluded from AHRI Standard 1250, it is not feasible to use AWEF to compare performance between the transcritical CO2 system and the baseline system. Instead, we conducted a direct Coefficient of Performance (COP) comparison between two systems running under the same conditions and capacity (AHRI 1250 “standard rating conditions for fixed capacity refrigerator matched-pair, dedicated condensing unit located indoor”) where only air-side conditions were specified. Since the baseline’s AWEF ratings were verified, the CO2 system was considered satisfactory, since its COP was equal to or higher than the baseline COP. CTS developed and built the CO2 demonstration unit in a lab test setting, and selected, designed, and evaluated the major components individually on a breadboard facility before packaging them for system testing. For a fair comparison, we set a design constraint for the outer packaged CO2 dimensions to be no larger than the baseline unit. After a series of design refinements and operating condition optimization, the CO2 demonstration unit tested as having a 2.01 COP and 5693 Btu/h (1.67 kW) net cooling capacity under AHRI 1250 standard rating conditions, which matches the baseline 1.99 COP and 5659 Btu/h (1.66 kW) capacity. In addition to matching performance to the baseline, the CO2 demonstration unit was fully functional in extreme hot and cold outdoor temperatures of 112°F and 11oF, with a minor modification to the Internal Heat Exchanger (IHX). In considering potential benefits, we expected the CO2 system’s hardware cost to initially to be higher than conventional refrigeration systems, due to the major components’ limited market availability. However, it would be reasonable to anticipate a significant CO2 unit cost reduction as market volume increased. Since the CO2 system’s level of complexity was comparable to conventional system design, it would be reasonable to assume future cost differences between CO2 systems and those using conventional technology could become negligible for this type of system. This study also demonstrated the CO2 system’s refrigerant cost was only 1/46 of the R448A system, and the Greenhouse Gas (GHG) quantity was also significantly reduced. These long-term benefits after initial investment should greatly increase the transcritical CO2 walk-in system’s market attractiveness.