Study and development of low-GWP mixtures for commercial condensing units and ultra-low temperature systems

Abstract

This PhD thesis develops low- or negligible-GWP refrigerant mixtures to improve energy efficiency and reduce refrigerants’ GWP in two key areas: medium-capacity CO2 systems and ultra-low temperature (ULT) autocascade systems. For CO2 systems, small additions of secondary refrigerants enhance energy performance. Experimentally, the mixture CO2/R152a achieved up to 11.2% COP improvement in basic cycle, whereas with advanced configurations the improvements were around 3%. With parallel compression cycle, new separation phenomena are unveiled. In the ULT segment, the thesis presents the first full thermodynamic analysis of an autocascade system using a novel calculation method based on liquid density. The optimal mixture (R600a/R1150 [70/30%]) and system enhancements, such as a capillary optimization, compressor replacement and recuperative low-temperature heat exchanger, increased COP from 0.144 to 0.338. Overall, this work proposes efficient, lower-impact alternatives to high-GWP refrigerants in two critical application segments.