Energy improvements of CO2 transcritical refrigeration cycles using dedicated mechanical subcooling

In this work the possibilities of enhancing the energy performance of CO₂ transcritical refrigeration systems using a dedicated mechanical subcooling cycle are analysed theoretically. Using simplified models of the cycles, the modification of the optimum operating conditions of the CO₂ transcritical cycle by the use of the mechanical subcooling are analysed and discussed. Next, for the optimum conditions, the possibilities of improving the energy performance of the transcritical cycle with the mechanical subcooling are evaluated for three evaporating levels (5, −5 and −30 °C) for environment temperatures from 20 to 35 °C using propane as refrigerant for the subcooling cycle. It has been observed that the cycle combination will allow increasing the COP up to a maximum of 20% and the cooling capacity up to a maximum of 28.8%, being both increments higher at high evaporating levels. Furthermore, the results indicate that this cycle is more convenient for environment temperatures above 25 °C. Finally, the results using different refrigerants for the mechanical subcooling cycle are presented, where no important differences are observed.     

Optimization of a CO2–C3H8 cascade system for refrigeration and heating

Conventional working fluids (refrigerants) are being phased out worldwide to combat with the twin menace of ozone layer depletion and global warming and natural refrigerants are fast gaining favour lately. Single stage and multi stage refrigeration systems fail to widen the gap between heat source and heat sink temperatures required in many industrial applications requiring simultaneous heating and cooling and cascaded systems appear to be the best alternative. Modest research has been done in cascaded systems based on natural refrigerants thereby offering good potential for research. In this paper, a cascaded system for simultaneous heating and cooling (refrigeration and heat pump system) with a carbon dioxide based HT cycle and propane based LT cycle for simultaneous refrigeration and heating applications has been analyzed. To facilitate prediction of optimum performance parameters, performance trends with variation in the design parameters and operating variables have been presented in this article. Relevant expressions have been developed to serve as guidelines to the user for selecting appropriate design parameters like intermediate temperature so that the system yields optimum performance. Independently developed property codes have been employed for both carbon dioxide and propane for higher accuracy.     

Thermodynamic analysis of optimal condensing temperature of cascade-condenser in CO2/NH3 cascade refrigeration systems

This study thermodynamically analyzed a cascade refrigeration system that uses carbon dioxide and ammonia as refrigerants, to determine the optimal condensing temperature of the cascade-condenser given various design parameters, to maximize the COP and minimize the exergy destruction of the system. The design parameters include: the evaporating temperature, the condensing temperature and the temperature difference in the cascade-condenser. The results agreed closely with the reported experimental data. The optimal condensing temperature of the cascade-condenser increases with T_C, T_E and ΔT. The maximum COP increases with T_E, but decreases as T_C or ΔT increases. Two useful correlations that yield the optimal condensing temperature of the cascade-condenser and the corresponding maximum COP are presented.     

Experimental investigation on the performance of NH3/CO2 cascade refrigeration system with twin-screw compressor

This paper describes the experiment carried out to analyze the performance of a refrigeration system in cascade with ammonia and carbon dioxide as working fluids. The effect of operation parameters, such as the evaporating temperature of the low temperature cycle, the condensing temperature of low temperature cycle, temperature difference in cascade heat exchanger and superheat degree, on the system performance was investigated. Performance of the cascade system with NH₃/CO₂ was compared with that of two-stage NH₃ system and single-stage NH₃ system with or without economizer. It was found that the COP of the cascade system is the best among all the systems, when the evaporating temperature is below −40 °C. Also, the cascade system performance is greatly affected by evaporating temperature, condensing temperature of low temperature cycle, temperature difference in cascade heat exchanger and is only slightly sensitive to superheat degree. All the experimental results indicate that the NH₃/CO₂ cascade system is very competitive in low temperature applications.     

Energy and exergy comparison of a cascade air conditioning system using different cooling strategies

A cascade air conditioning system consisting of a compression and an absorption chiller working in a parallel arrangement has been proposed. This system is powered up by a micro-gas turbine. Different cooling strategies are studied in order to recognize the best configuration. The system components have been modeled and analyzed through the energy and exergy approaches. The performance parameters of the systems and the second law efficiency have been calculated in different operating conditions. Water consumption of the systems has also been investigated, considering water as a source of exergy. The results revealed that a system with water-cooled chillers has the highest second law efficiency and water consumption. On the other hand, the water consumption of a system with an air-cooled absorption and a water-cooled compression chiller is about 50% less than that of the system with two water-cooled chillers while its second law efficiency is only about 10% less.     

Exergy maximization of cascade refrigeration cycles and its numerical verification for a transcritical CO2–C3H8 system

Analysis of an endoreversible two-stage cascade cycle has been implemented and optimum intermediate temperature for maximum exergy and refrigeration effect have been obtained analytically. Further, the heat reservoir temperatures has been optimised independently. A comprehensive numerical model of a transcritical CO₂–C₃H₈ cascade system was developed with intent to verify the theoretical results. It is seen that the simulation results agree well for optimal T_L but deviate modestly from the theoretical optimum of T_H. It has also been observed that system performance improves as T_H increases and unlike theoretical predictions, no optimal T_H is present within feasible working temperatures.