Alternative refrigerant mixtures to Isobutane (R-600a): energy efficiency enhancement and flammability reduction

Abstract

This doctoral thesis explores alternative refrigerant mixtures to isobutane (R-600a) for vapor compression refrigeration systems, aiming to improve energy efficiency and reduce flammability. Isobutane, widely used in Europe for domestic and low-capacity commercial refrigeration due to its low global warming potential (GWP) and good energy properties, is limited by its high flammability, restricting the maximum refrigerant charge and cooling capacity. This research identifies potential refrigerant blends that balance energy efficiency, volumetric capacity, low GWP, and flammability. A comprehensive thermodynamic analysis of 120 ternary blends of 10 refrigerants reveals promising mixtures such as R-1270/R-600, R-152a/R-600, R-1234yf/R-600, R-1234ze(E)/R-600, and R-290/R-600, which show significant increases in COP compared to R-600a. A new method created to theoretically predict the flammability class of new blends highlights the challenges in significantly reducing flammability by mixing hydrocarbons with less flammable refrigerants. Experimental tests on domestic and commercial refrigeration units demonstrate energy consumption savings with blends like R-1234yf/R-600a (7.5/92.5)%, R-1234ze(E)/R-600 (10.5/89.5)%, and R-290/R-600 (11/89)% in domestic units, and R-1234ze(E)/R-600 (8/92)% and R-152a/R-600 (8/92)% in commercial units. Optimal compositions for R-152a/R-600 and R-290/R-600 mixtures, tested on a specially constructed bench, show COP increases of 7.3% and 10.3%, respectively. The thesis concludes that certain binary blends can improve energy efficiency and maintain system compatibility but they cannot simultaneously achieve reduced flammability, increased efficiency, and low GWP.