Abstract
Different alternatives are being studied nowadays in order to enhance the behavior of transcritical CO2 refrigeration plants. Among the most studied options, subcooling is one of the most analyzed methods in the last years, increasing cooling capacity and Coefficient Of Performance (COP), especially at high hot sink temperatures. A new cycle, called integrated mechanical subcooling cycle, has been developed, as a total-CO2 solution, to provide the subcooling in CO2 transcritical refrigeration cycles. It corresponds to a promising solution from the point of view of energy efficiency. The purpose of this work is to present, for the first time, thermodynamic analysis of a CO2 refrigeration cycle with integrated mechanical subcooling cycle from first and second law approaches. Using simplified models of the components, the optimum operating conditions, optimum gas-cooler pressure, and subcooling degree are determined in order to obtain the maximum COP. The main energy parameters of the system were analyzed for different evaporation levels and heat rejection temperatures. The exergy destruction was analyzed for each component, identifying the elements of the system that introduce more irreversibilities. It has been concluded that the new cycle could offer COP improvements from 11.7% to 15.9% in relation to single-stage cycles with internal heat exchanger (IHX) at 35 °C ambient temperature.
Highlights
The refrigeration sector has been highly altered in recent years due to the latest European directive [1] and other restrictions and protocols [2,3], which leave carbon dioxide as the only alternative for centralized commercial refrigeration because of its low Global Warming Potential (GWP) and its security characteristics
The results presented on the paper correspond to the evaluation a single-stage CO2 refrigeration cycle with integrated mechanical subcooling cycle, based on manufacturers data
The use of an integrated mechanical subcooling system for improving CO2 refrigeration systems is studied from a theoretical approach
Summary
The refrigeration sector has been highly altered in recent years due to the latest European directive [1] and other restrictions and protocols [2,3], which leave carbon dioxide as the only alternative for centralized commercial refrigeration because of its low Global Warming Potential (GWP) and its security characteristics (non-flammable nor toxic, A1 ASHRAE classification). CO2 working in classical refrigeration cycles has some inconveniences such as its low performance compared to systems working with other HydroFluoroCarbons (HFC) refrigerants. This is the reason that the greatest technological advances in the last years have been developed in line with the search for solutions to improve the performance of this refrigerant in hot climates, where classical configurations do not perform well enough. Some research lines have proposed the use of a parallel compressor in the system to improve the energy behavior. The cycle with parallel compression economization reached improvements in COP of 47.3% in relation to the basis
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