Performance analysis of a dual-ejector enhanced two-stage auto-cascade refrigeration cycle for ultra-low temperature refrigeration

Auto-cascade refrigeration cycle (ARC) is a cost-effective solution for ultra-low temperature refrigeration. This paper proposes a dual-ejector enhanced two-stage auto-cascade refrigeration cycle (ETARC). In ETARC, two ejectors replace the throttle valves at the liquid phase outlet of the two gas-liquid separators in the conventional two-stage auto-cascade refrigeration cycle (CTARC). They are connected in series to recover partial expansion work in the throttling process, aiming to lift the suction pressure of the compressor. The ternary mixture component is selected and R600a/R41/R1150 is supposed to be the working fluid due to the best performance. The mass fraction ratio of the refrigerant mixture components, the quality at the inlet of two separators and the expansion ratio of the throttle valve (EPR) of the two cycles are optimized using particle swarm optimization (PSO) with respect to the maximum COP. The performances of the two cycles at optimum operating conditions are compared using energetic and exergetic methods, and the effect of the evaporation temperature and the condensation temperature on cycle performances and operating characteristics are discussed in detail. The results show that applying the two ejectors effectively improves the performance of ETARC. Compared with the CTARC at the evaporation temperature of -85 degrees C, and the condensation temperature of 45 degrees C, the ETARC cycle exhibits 16.07% improvement in COP, 29.43% increase in the volumetric refrigeration capacity qv, 17.17% reduction in the total exergy destruction rate. ETARC shows significant performance improvement and has a remarkable energy-saving potential.