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.

An experimental comparison has been performed of a cascade refrigeration facility working with therefrigerant pairs R134a/R744 and R152a/R744. This kind of facility is suitable for industrial and commercial refrigeration applications. The high GWP refrigerant R134a has been substituted with the low GWP refrigerant R152a, in accordance with the new environmental regulations aimed at mitigating the Greenhouse effect. As both refrigerants belong to the family of HFC fluids, the replacement has been carried out as a drop-in. Apart from safety considerations, as R152a is included in the A2 group, the results of the wide range of tests conducted show that no special energy improvement or worsening is achieved, and that the replacement of R134a with R152a is technically and energetically feasible. KEYWORDS

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This work analyses and quantifies the effects caused by the use of an internal heat exchanger (IHX) at the CO2 subcritical cycle in an HFC134a/CO2 cascade refrigeration plant that incorporates a gas-cooler at the exit of the low temperature compressor. Previous theoretical and experimental studies showed that the IHX reduces the refrigeration capacity and COP of the subcritical cycle, however, it has been seen that it also lowers the heat to be rejected at the condenser. This reduction, when the cycle is a part of a cascade system, allows reducing the heat load of the high temperature cycle, modifying the working conditions of the cascade plant. The modifications result in an increment of the overall coefficient of performance of the cascade system. The analysis here presented is based on the evaluation of an experimental HFC134a/CO2 refrigeration plant, which has been analysed with and without internal heat exchanger in an evaporating temperature range from -40 to -30 ºC and in a condensing one from 30 to 50 ºC. The plant incorporates a gas-cooler at the exit of the CO2 compressor. The experimental results confirm that the IHX slightly reduces the cooling capacity but it can increment the overall COP up to 3.7 %.

Energy improvements offered by dedicated and integrated mechanical subcooling systems in CO 2 booster systems for supermarket applications are analysed here through the use of thermodynamic models close to reality. Using a reference supermarket with 41kW and 140kW thermal loads at low and medium temperature, respectively, and considering as state-of-the-art system the CO 2 booster with parallel compressor and flash gas by-pass, it has been concluded that both systems allow to reduce energy consumption. However, its operation is highly dependent on environmental conditions. The dedicated mechanical subcooling system offers annual energy reductions for tempered places from 1.5 to 2.9%, for warm between 2.9 to 3.4% and for hot from 3.0 to 5.1%. The integrated subcooling system obtains reductions between 3.1 to 4.0% for cold regions, from 1.4 to 2.9% for tempered, from 2.9 to 3.4% for warm and from 1.3 to 2.4% for hot regions.