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CO2 transcritical refrigeration cycles require optimization to reach the performance of conventional solutions at high ambient temperatures. Theoretical studies demonstrated that the combination of a transcritical cycle with a mechanical subcooling cycle improves its performance; however, any experimentation with CO2 has been found. This work presents the energy improvements of the use of a mechanical subcooling cycle in combination with a CO2 transcritical refrigeration plant, experimentally. It is tested the combination of a R1234yf single-stage refrigeration cycle with a semihermetic compressor for the mechanical subcooling cycle, with a single-stage CO2 transcritical refrigeration plant with a semihermetic compressor. The combination is evaluated at two evaporating levels of the CO2 cycle (0 and -10 ºC) and three heat rejection temperatures (24, 30 and 40 ºC). The optimum operating conditions and capacity and COP improvements are analysed with maximum increments on capacity of 55.7 % and 30.3 % on COP. KEYWORDS

CO 2 subcooling has resulted a method to upgrade the performance of CO 2 refrigeration plants in the recent years, with overall improvements up to 12% with internal heat exchangers, 22% with economizers, 25.6% with thermoelectric systems and 30.3% with dedicated subcooling methods. This paper comprehensively reviews the recent studies that consider subcooling as a way to upgrade the performance of CO 2 refrigeration cycles. The review is limited to CO 2 refrigeration cycles with accumulation receiver for commercial purposes and does not consider air conditioning or MAC systems. It is organized as follows: first, the thermodynamic aspects of subcooling in CO 2 refrigeration cycles are described and discussed; second, the main results and conclusions of the recent investigations are analysed inside two big groups: subcooling internal methods and subcooling external methods. Finally, the review synthesizes the current state of the art and points out the lines of research that deserve future developments. Highlights Research using subcooling as way to improve CO 2 refrigeration is analysed. COP improvements up to 37.8% of CO 2 base systems have been reported. State-of-the art subcooling systems are presented and discussed  New opportunities for research are highlighted in the review.

This work analyses different refrigeration architectures for commercial refrigeration providing service to medium and low temperature simultaneously: HFC/R744 cascade, R744 transcritical booster, R744 transcritical booster with parallel compression, R744 transcritical booster with gas ejectors, R513A cascade/R744 subcritical booster, and R513A cascade/R744 subcritical booster with parallel compression. The models were developed using compressor manufacturers’ data and real restrictions of each system component. Limitations and operating range of each component and architecture were analysed for environment temperatures from 0 to 40 °C considering thermal loads and environment temperature profiles for warm climates. For booster systems, cascade with subcritical booster with parallel compression provide highest coefficient of performance (COP) for temperatures below 12 °C and above 30 °C with COP increases compared basic booster up to 60.6%, whereas for transcritical boosters, architecture with gas ejectors obtains the highest COP with COP increases compared to the basic booster up to 29.5%. In annual energy terms, differences among improved booster systems are below 8% in the locations analysed. In Total Equivalent Warming Impact (TEWI) terms, booster architectures get the lowest values with small differences between improved boosters.

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