Theoretical analysis of three CO2/C3H8 (R744-R290) cascade refrigeration systems with precooling processes in low-temperature circuits

[Display omitted] •Three CO2/C3H8 cascade refrigeration systems with precooling processes are proposed.•Performance of three systems is comparatively studied with a conventional system.•Potential of new systems are evaluated for component capacity and applicable condition.•Three proposed systems perform better in thermodynamic performance. In a cascade refrigeration system (CRS), the superheat of the vapour discharged from a low-temperature compressor is large for refrigerants with significant adiabatic exponents, and part of the cooling heat in the fluid can be removed using precooling technology. In this study, three CO2/C3H8 cascade refrigeration systems with precooling processes, namely a CRS with an air precooler (CRSA), CRS with a mechanical precooler (CRSM) and CRS with both an air precooler and a mechanical precooler (CRSAM), are proposed. The performances of the proposed systems are compared studied that of a conventional cascade refrigeration system (CCRS) in terms of energy and exergy. The results indicate that a design that arrange precoolers and precooling refrigerant circuits in low-temperature circuits is beneficial, directly reducing the work input of C3H8 compressors and improving refrigeration performance. When the condensing temperature rises from 12 °C to 48 °C, the coefficient of performance (COP) of the CRSA, CRSM and CRSAM enhances by 2.73% to 5.19%, 1.82% to 4.90% and 3.71% to 7.86% compared with the CCRS, and the exergy efficiency averagely improves by 4.01%, 3.41%, and 8.48%, respectively. In addition, the applicable conditions of the different systems are discussed, and the component cost is evaluated in terms of component capacity. The research results may provide guidelines for the practical application of CO2/C3H8 CRS with precooling processes.