Theoretical analysis of a CO2–NH3 cascade refrigeration system for cooling applications at low temperatures

As a result of environmental problems related to global warming and depletion of the ozone layer caused by the use of synthetic refrigerants (CFC’s, HCFC’s and HFC’s) experienced over the last decades, the return to the use of natural substances for refrigeration purposes, appears to be the best long-term alternative. In this paper, a cascade refrigeration system with CO2 and NH3 as working fluids in the low and high temperature stages, respectively, has been analysed. Results of COP and exergetic efficiency versus operating and design parameters have been obtained. In addition, an optimization study based on the optimum CO2 condensing temperature has been done. Results show that following both method’s exergy analysis and energy optimization, an optimum value of condensing CO2 temperature is obtained. The compressor isentropic efficiency influence on the optimum system COP has been demonstrated. A methodology to obtain relevant diagrams and correlations to serve as a guideline for design and optimization of this type of systems has been developed and it is presented in the paper.