CO2, R-513A, Booster, Transcritical, Hydrofluoroolefin
As global regulatory framework has been introduced to phase down the manufacture and use of hydrofluorocarbon (HFC) refrigerants on a total carbon emissions basis, many end users have been tasked with finding alternatives to current refrigeration architectures and associated working fluids. Using commercial refrigeration as an example, transcritical R-744 (CO2) booster architectures have been studied, piloted, and compared to conventional R-404A centralized systems. Many studies show favorable energy consumption and COP (coefficient of performance) in colder climates for CO2 versus R-404A. Concurrent with the advances in CO2 technology is the development of the lower GWP (global warming potential) HFC alternative of hydrofluoroolefins (HFO). This paper discusses the development and use of thermodynamic cycle models for a transcritical CO2 booster system and an R-513A booster system in commercial refrigeration. Base system architectures were compared as well as upgrades to these systems including parallel compression, ejectors, and adiabatic condensers. Optimization schemes were used for system COP, gas cooler pressure, flash tank intermediate pressure, and ejector motive nozzle outlet pressure. A correction factor was applied for the medium and low temperature case loads to account for the influence of outdoor air temperature (OAT). Additionally, while a standard approach temperature was used for the modeled R-513A condenser, CO2 gas cooler outlet temperatures were calculated using published experimental correlations to OAT to provide a fair comparison. Weather data from twelve select cities of differing climate zones and moisture regimes in the United States were used in conjunction with a selected set of system inputs representing typical commercial refrigeration operation. Comparison plots of both total compressor energy consumption and combined system COP as a function of ambient temperature were used to compare the various architectures. Total yearly compressor energy consumption by system type at each of the studied locations was also reported.