Adverse effects of synthetic refrigerants on the environment have led to replacing them with natural refrigerants. The common candidates are ammonia, carbon dioxide, and several hydrocarbon compounds and their mixtures. Ammonia has been used mainly in large-scale cooling purposes such as large-scale supermarkets and climatic rooms. However, in such systems, leakage of ammonia may arise severe results on human health and may damage products in the cooled space. Recently, in last decade, a well-known refrigerant, CO2, has gained more attention to be applied in refrigeration systems due to having prominent thermo-physical properties. The performance analysis of a CO2/NH3 cascade (CAS) system has been theoretically examined in the current study. The detailed performance analysis of the system and optimization of the operating parameters have been studied extensively. In addition, the second-law analysis of the system with both cycles has been performed. Optimum operating conditions of the system are also determined and correlations are developed. Finally, the coefficient of performance (COP) correlations developed by several researchers in literature and those of current study are compared against available experimental COP results. The comparisons showed that the proposed correlations can be utilized for the accurate prediction of the COP of a cascade CO2/NH3 system within the studied range of operating conditions.
In this study, performance of two different environmentally friendly systems with natural refrigerant solutions, R744 transcritical booster system with ejector and a R290/R744 cascade system are examined theoretically by using engineering equation solver software. Operating conditions are determined to represent different climatic regions in Turkey using summer dry bulb temperatures of various cities. The transcritical and the cascade system are assumed to operate at two different evaporation temperatures of-10 °C and-32 °C. The overall energy efficiency ratio values for each system with respect to the same ambient and evaporation conditions are compared and evaluated. Finally, performance of both systems has been compared and the appropriate solution for each city has been suggested. For cold and mild climate regions of Turkey, the performance of transcritical alternative is found better than the proposed cascade system. Moreover, the performance of transcritical system is observed slightly lower than those of the cascade system in hot climate regions of Turkey such as Aegean, Mediterranean, and SouthEast Anatolian regions. It is also found that the performance of the transcritical system is better in regions having lower ambient conditions such as near the Black Sea and eastern regions of Turkey. Therefore, for the mild and cold regions of Turkey, the transcritical ejector option is the better alternative due to having higher performance compared to the cascade system.
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